In the SEC, it is bigger. That is why NFL franchises like the Saints, Falcons, Bucs, Jaguars, etc. leave in the shadow of their SEC counter-parts in state. That is why the ACC, B1G, and Pac12 is having to make an ALLIANCE against the SEC right now. You can argue that the SEC isn't any better, especially in mid and bottom tier, than the other leagues but you have a hard time arguing fanbase. Only the B1G competes and even they have states that see NFL teams far stronger than College.
Look at a team like Arkansas. They are the only game in town for that state and they have a very large fan base despite their record.
Iowa State lives in the shadow of Iowa, that is their problem. I have been to Iowa several times and 2/3 of the CFB fans that I have met are Hawkeyes.
This is the REASON the Big12 doesn't have any suitors. If Iowa State have an SEC level fanbase, they would be a lock for a P5 league.