I'm not saying they don't.
USC and UCLA have tons of fans because they're in LA, and LA has tons of people to split between only two college football teams.
BYU and Utah will eventually reach an equilibrium where they have more or less the same number of local fans. But, BYU will always have significantly more fans outside of Utah because it's the only Mormon school with sports, and most of its students will be out-of-staters.