There are many good anime series that could use additional seasons of material. Bastard, for instance. Or Guyver. But these shows often have dismal sales in Japan. In order to make a sequel to these series, they'd probably have to mark it all up to charity, because they'll never get back the cost of production.
There are other shows that made a small profit, but the industry norm is for sequels to have fewer sales than the original season's average. It can drop anywhere from 20% to 50% each time a new season is released. This is because nobody new can enter the series unless they've watched the previous season, but it's still possible to get tired of the new material even when you were content with the original. It's an ever downward slope with no potential for progress. This means a new season to a show is a pretty frightening prospect for shows with only mild profitability. The break-even point for most anime is 3,000 in sales, the fabled 'manabi line.' So if sequels can often sell only half as much as the previous season, that previous season needs to be at 6,000 for producers to have absolute confidence to move forward with a new season.
There are of course exceptions to this rule. If a show is trying to promote a video game, merchandise, or something else, it's possible to make a profit even with lower disk sales. For instance, a White Album 2 anime was made even though the original White Album anime had dismal sales, because they wanted to promote the White Album 2 visual novel and thought the low sales a worthwhile risk. Also, in some rare cases sequels to shows have actually done better than their previous season, like with Symphogear or Nanoha. But you can hardly rely on such miraculous results. Generally speaking, you want a large comfortable margin of success before you okay a new season, and 6,000 is a wide enough margin of success that nothing can go wrong.
As you might have guessed, great series like Da Capo III, Btoom! and Sakura Trick all failed to reach this magical number of 6,000 sales. That's why they weren't given a sequel. No matter how distinguished their records up until that point, they were no longer reliable money-makers from here on. It was time to abandon the franchises and search for greener pastures.
However, this doesn't explain the series that have over 6,000 copies in average blu-ray/dvd sales not receiving a sequel, and there's quite a few good ones that have been inexcusably overlooked. When a studio has a chance to release a really great anime and make a surefire profit when it does so, there's really no excuse left for why these sequels haven't at least been announced:
K-On!!'s sales were at 39,385, and yet they still haven't announced a sequel that could actually catch the series up to where the manga ends. There's literally no reason for this absurdity. How often do you have a franchise with 40k sales and more source material and you don't announce a sequel? That makes no sense. Good grief.
Lucky Star's sales were at 29,146, and yet they still haven't announced a sequel when there's plenty of manga material left. Yet again Kyoto Animation discards a series with infinite sales potential to the everlasting void.
Mahou Shoujo Lyrical Nanoha StrikerS' sales were at 22,591, but instead of the perfectly reasonable adaption of their Vivid or Strike Force mangas for a new season, they chose to do some theatrical releases and otherwise simply ignore this high flying franchise. What sense does this make?
Haruhi Suzumiya (2009)'s sales were 19,052, even though this included the abhorrent endless 8 section of the story. If Kyoto Animation were to take their job seriously and stop playing pranks on their viewers, the sales could be even higher for this franchise. Even so, Kyoto decided to abandon this money maker with plenty of light novel source material left to oblivion. Noticing a pattern yet?
The second season of Index got 16,994 in sales, and the second season of Railgun got 16,664. It would make sense to announce either of these series for a sequel, or even both at once, but instead all we get is silence. How could such a successful franchise be so neglected? It's been over a year since the last Railgun episode came out. There's plenty of Index light novels and Railgun manga left to adapt. What are they waiting for?
Hataraku Maou-sama! got 11,511 in sales. There's plenty of light novel material left. This should be a no-brainer. I'm seriously confused why no sequel announcement has been made yet for this show.
To Love-ru Darkness sold 10,784 copies. Perhaps they're waiting for more of the manga to be released first, but in any case it's obvious that the anime should continue to adapt the manga so long as it has great sales like this.
Hyouka's sales are at 9,795. There's still one more light novel left to adapt from the franchise. Why not make a movie or oav series and cover this one last bright opportunity? It would be sure to sell well. Too bad Kyoto Animation did this series as well, as there's nothing more reliable than Kyoto Animation letting down their fans.
Little Busters! Refrain has an average of 9,233 in sales. This is more than enough to prompt J.C. Staff to announce a sequel Key adaption -- whether it's Kud Wafter, the direct followup to Little Busters, or Rewrite, the spiritual successor, in any even it's high time for more Key anime to be produced.
No Game no Life scored a blistering 8,860 sales for this latest spring season. 2014 has had very few hits, so this should be a no-brainer green light for a sequel. There's plenty of light novels to go, so Madhouse should hurry up and announce the good news that they will indeed adapt them. What's to lose?
Utawarerumono sold 7,776 copies. When the Utawarerumono 2 video game comes out, the anime could be used to promote it, while also getting a ton of profit for itself. I admit the game hasn't been released yet, but there'd be no harm in announcing the intention of doing the sequel anime now and actually producing it later, right?
Tales of the Abyss received 7,498 sales. This is the best a Tales series has done, and Abyss isn't even a particularly famous or good game compared to Vesperia or Xillia. Why not take the plunge and adapt some more games into an anime when Abyss did so well? Vesperia is the most obvious choice, but any number of other games would also be cool, from virtually anywhere in the long franchise's history, every single adaption would be welcomed by fans. At least keep adapting the video games into anime until you suffer a corporate loss as a result. With 7,498 in sales, what is there to fear?
Kimi ni Todoke 2nd Season got 6,776 in sales. The manga is still wildly popular. I'm sure a third season would be welcomed by fans with open arms. Why stop midway? I can understand if the anime makers want to wait until the manga is finished so they can deliver a satisfying closure to the show, but they could do that for a 4th season or 5th season. There's already plenty of material for a third season.
High School of the Dead got 6,684 in sales. Granted, the manga just randomly cuts off and the author has been on hiatus with the series for years as he writes Triage X instead. Granted, even if the anime continues it would just hit a new roadblock a few episodes later. But even so, what's the harm in adapting what manga we have? Even if it's just a movie or an OAV series, adapting the rest of High School of the Dead can't possibly go wrong.
Mikakunin de Shinkoukei got 6,304 in sales. I don't know how much more manga there is left to adapt, but there would be no harm in announcing the sequel season now and producing it later, right? Such a big hit should be followed through with from such a great start.
Claymore got 5,178 in sales even though its ending was so mutilated by wretched filler nonsense. Imagine how high its sales could go if it followed the manga? With the manga nearing its conclusion, the anime could restart where it first deviated from the manga and tell the story right this time, and reap the rewards of being faithful to the series' fans.
Spice and Wolf 2 got 5,096 in sales even though they still haven't reached the all important endpoint of their journey in the north. Now that the show can actually move the plot forward and continue until the end of the light novels, delivering a full and satisfying conclusion to the entire tale, is there any doubt it won't sell at least as well?
Full Metal Panic! Second Raid got 4,962 in sales. Considering how legendary a series this is and how much love people still have for it after all these years, I doubt the sales figures of a new season would slip any further down than this figure. In fact, I bet fans would reward the sequel with a boom in popularity and the sales would actually increase. A new FMP season could go all the way to the conclusion of the riveting series because the light novels are already long finished, and series with proper conclusions will definitely generate an uplift in sales compared to a useless cliffhanger ending like we currently have. The only problem is that, yet again, Kyoto Animation made this series, so like usual we can expect to be betrayed and abandoned by anything they get their grubby paws on.
Saki Zenkoku-hen got 4,719 in sales. This might seem low, but then again it was covering a rather dull portion of the manga. A mere quarterfinal game where Kiyosumi blew through their competition like paper. If the manga ever finishes (and I know it's a big if, given how long the author is taking), the story will be about the much more exciting semifinal and finals matches we've all been waiting for. The moment when Achiga faces off against Kiyosumi in the final will be one of the greatest cinematic events in history! There's no way the fans wouldn't bite. A sequel doesn't have to be announced right away. But by the time the manga is ready to finish, this series absolutely has to have a sequel to show the final flourish in proper animated fashion.
Oreshura sold 3,887, Baka to Test ni sold 3,800, Maria-sama ga Miteru 4th season sold 3,649, PapaKiki sold 3,470, Baccano sold 3,389, and Chihayafuru 2 sold 3,235. These numbers aren't great, but they are above the manabi line. It's just as likely that a sequel to these shows will make a profit as not, and the series are so famously good that isn't it worth taking a risk on them just in order to produce good art? A sequel to these shows will probably make money overall. The median 2014 show produced this year hasn't made a profit. (23 shows above the manabi line, 39 below) A sequel to these reliable money-makers is still a better option than simply gambling on completely new shows with no track record behind them. Why not give them another chance?
If Maria-sama were given a sequel, it could show Touko becoming a petit souer and Sachiko graduating, making Yumi the top ranking Rosa Chinensis. Even Yoshino would get to make Nana Arima her petit souer, which would complete the cycle of all the original first years moving on to become grande souers. That's a lot for fans to love.
Papa Kiki just has unbounded potential. Sasha's return from Russia and reconciliation with Miu, Hina's 4th birthday, Shiori's introduction and Sora standing up for her Oniichan, Hina's dog dying, there's basically endless material to call upon. The latest ova of PapaKiki was fantastic. Surely fans would willingly pay for more.
Baka to Test in the light novels has reached its conclusion. This means the 3rd season would have the juiciest part of the story to tell -- the resolution of all the romance pairings. Who wouldn't buy a copy to see that long awaited moment? It also has Himeji's backstory of when she was in Elementary school with Akihisa, and how exactly she fell in love with him in the first place. It's an excellent story that fills in the last missing piece to the romantic puzzle shown in the beginning of the first season.
Chihayafuru has more Karuta and Baccano surely has more gang wars, so what's there to dislike? There's a huge fan following for both series and I'm sure they're eager for more by now.
*I assume Shingeki no Kyojin will be receiving a sequel as soon as there's enough manga out to support it, so I didn't even worry about lobbying for it.
*I also assume Monogatari season 3 will come out after the Kizumonogatari movie is finally released, so I'm not yet worried about this show randomly being cancelled.
I'm not asking for charity here. Any of the series above listed would keep food on the table for the studios involved. I guarantee success for every one of them. So go ahead and adapt these shows, Japan. No excuses. We're all waiting on you.