The stadium is sized to the demand, it's not limiting it - they just expanded it a couple years ago and only added 3500 seats, and they haven't had a capacity crowd at the new size yet. And their TV money being so low isn't exactly a mark in their favor. Even if Boise was responsible for every dime of MWC TV money, they'd still be brining in less than the average per team payout in the Power 5.
They just aren't as valuable of a football property as their level of national exposure would lead you to believe, and it's because of their location. Now, I'm sure there are conferences that would be better off from a revenue perspective by booting an existing school and bringing in Boise, but that's whole different animal. They just don't bring enough to the table to expand the pie enough for a Power 5 conference. If they did, someone would be working around any other factors.