It really doesn't have that much to do with football, either.
Sometime in the 90s, local students became a minority at BYU. As soon as that happened, the trend we're seeing now became inevitable.
Very few kids growing up in Utah today will go to BYU. We're getting to a point now where most if their parents didn't go to BYU either.
Football is cyclical. Fan interest will wax and wane accordingly. But, there will never again be as many BYU fans in Utah as there were in the 80s, because BYU stopped being a "Utah" school a generation ago.
BYU still has plenty of fan growth. But it will probably never again come from in-state.