The Sun Belt is an up and coming conference with lots of teams that are doing well against outside competition. Even if they don't beat power 5 opponents, they often put quite a scare into them, running the game down to the wire.
As a result, it makes sense that they're looking for a way to polish their brand. The second and third worst teams in college football history in terms of win percentage, Idaho and New Mexico State, are being kicked out of the Sun Belt conference for just being too damn weak.
In addition, the teams are thousands of miles away from the rest of the conference, which is all located in the same Dixie footprint otherwise. Geographically cohesive conferences are always better than random assortments of teams with no historical connection to each other.
Considering their win loss record to date in the FBS, it's obvious both these colleges need to either drop football altogether or seek weaker competition in the FCS leagues. Maybe they could become competitive again in a region more suited to their nature.
The Sun Belt will fall down to 10 members, but ten healthy programs is better than 10 healthy programs + 2 sick ones who drag the rest down, so this is pretty much a no-brainer.
All these changes are slated for 2018, so we'll just have to see how the FBS morphs from here. I don't think there's been a single year so far where two consecutive seasons had no changes in conference alignment in the FBS in the past decade. The turbulence continues. Nobody knows what will become of UMass once it leaves the MAC either.