The anime industry was recently cited to the effect that you would need to sell over 6,000 copies of each volume blu-ray or dvd release to make a profit without any other source of revenue. Normally, anime does have other sources of revenue, like music, merchandise, and manga tie-in sales. But let's just take that 6,000 number to heart. In other words, if your anime sold over 6,000 copies per volume last time, then there shouldn't be any reason not to make a sequel, since the last time it was profitable, right? In other words, there's no excuse for anime industries neglecting series that sell 6,000 copies per volume no matter how cutthroat the industry may be.
I suppose you can make the argument that even though last time it was profitable, next season will see an over 50% drop in sales as viewers lose interest and walk away from the previously profitable enterprise. This would require series to have 12,000 in sales each to 'guarantee' that nothing could go wrong. But this is ludicrous. The anime industry doesn't have enough 12,000 sales generators for this to be the exclusive source of new content. Numbers like that come once in a blue moon. Even 6,000 sales per volume is rare and reserved only for the most popular works. If anime required 12,000 sales per volume to be worth continuing, anime would have ended long ago. 6,000 is more than enough to make not only yourself, but also your weaker performing sequel profitable, on the basis of all the side benefits that go into anime production -- advertising for the original source, selling airtime to commercial advertisers, etc.
Obviously if the story is complete there's no chance of making a sequel, but most of the time the source work hasn't been finished so there's plenty of material left for another season. My objection is to when the anime industry drops the ball at these times.
So, with those objections discarded, why has the anime industry not made easily profitable and obvious decisions to make sequels to series that have met every possible requirement to be continued?
In 2011, Kimi ni Todoke 2nd season sold 6,776 copies per volume on average. It's the end of 2015 and still no sequel has been announced.
In 2010, K-On!'s average volume sales were 34,702 per volume. The anime ended at graduation, but the manga didn't. It followed them into their college career and Ui's high school life. Is there a reason not to cover the rest of the manga based material for such a wildly popular anime? Even assuming the rest of the manga was much worse than the original portion, we're talking 35k sales here. How much worse would it have to be to not make a profit?
In 2010, To aru Majutsu no Index II's average volume sales were 17,164 per volume. There's infinite more Index novels to adapt, and they aren't necessarily worse than the ones we've already seen. There is no conceivable reason why this anime hasn't received a sequel yet.
In 2009, Suzumiya Haruhi's 2nd season sold 19,052 per volume. There's plenty of additional Haruhi novel content to work with, and yet no 3rd season was ever announced. How can anyone justify this in economic terms?
In 2007, Claymore sold 6,016 copies per volume. This after they totally screwed up the 2nd half of the anime with an anime-original ending. There's plenty of manga left to adapt and clearly a loyal fanbase that will put up with practically anything and still buy the product, so why on Earth wasn't it given another season?
In 2005, Full Metal Panic the Second Raid sold 6,376 copies per volume. The books continued far beyond that point and we were heading towards a wonderful climactic finale to the whole epic spiel, but I guess that isn't good enough since no anime sequel was ever made.
In 2012, Hyouka sold 10,044 copies per volume. There's still one book left unadapted. Why not make an oav or movie about it?
In 2013, To aru Kagaku no Railgun S sold 16,923 per volume. This is hardly any different from Index's success. There's plenty of Railgun manga left to adapt as well. If for some reason you don't want to do Index anymore, couldn't you at least adapt the next arc of Railgun for a profit?
In 2013, Hataraku Maou-sama! sold 11,794 per volume. There's plenty of books left in the series. What is stopping the anime sequel from being announced?
In 2013, Little Busters Refrain sold 9,562 per volume. They could continue the story directly with Kud Wafter, or they could make a spiritual Key sequel by moving on to Rewrite. Why have they chosen to do neither of these things?
In 2014, SAO II sold 20,179 per volume. Alicization awaits. There's no reason why we couldn't be watching season three right now.
In 2014, Gekkan Shoujo Nozaki-kun sold 10,556 per volume. There's plenty of manga left to adapt. What's stopping them?
In 2014, No Game No Life sold 8,774 per volume. It has plenty of books left. So why doesn't it have an anime?
In 2014, Amagi Brilliant Park sold 6,789 per volume. The books wind on long after where the anime ended. Why was it ended?
In 2014, Mikakunin de Shinkoukei sold 6,362 per volume. The manga continues beyond this point, so why not the anime?
In 2015, Jojo's Bizarre Adventure Stardust Crusaders Egypt-hen sold 9,908 per volume. Why hasn't part 4's adaption been announced? We've still got so much more material to go.
In 2015, Hibike! Euphonium sold 9,304 per volume. The anime only covered the first book. There's lots of books to go. Please bring back my brass band.
There are many other shows with valid arguments for sequels who haven't received any. But these shows have gone above and beyond the line of duty to deserve a sequel. Their fans have done everything they could to ensure these shows' success, and as thanks they've been spat on and ignored. This is a betrayal. There's just no other terms for selling tens of thousands of copies per volume, only to not be given a sequel as thanks for your dishing out on all their previous products. 6 out of 17 of those betrayals all come at the hands of Kyoto animation, who categorically refuses to make sequels to popular works they've already begun. No one else is remotely as cruddy to their fans as Kyoto Animation is day in and day out, year after year. Talk about an abusive relationship.