There's crossover for sure. But if you've been to both an NFL and a College football game, you see the difference immediately. College football fans are alumni or family with typically deep history with that institution. Fair weather fans don't typically exist in college sports. Some, but not most.
If Utah ended up in one of the 2 main conferences, and BYU is out, are you going to root for Utah? Of course not. But regionally your theory says you should.
Different fans.