BYU grads are much more likely to come from out of state and to leave the state when they graduate.
The ratio of BYU grads to Utah grads in the state is going to trend toward Utah.
Also, I get that many people who don't attend either school may default to Utah. It is easier, in some ways. You can be Mormon, non-Mormon, and for some reason being a fan of Utah doesn't seem to tie you to the school's policies the way being a BYU fan does.
Winning would make a big difference. . .