is that you have a huge pool of people, often more educated than the average American, that all (mostly) want to go to the same school.
So you end up getting very high quality students given the high demand and very small supply. Not sure you can argue football changes that much. You still end up with a high volume of high-achieving students that want to go to BYU because it's an LDS school and experience, not because BYU football was glorious in the '80s and '90s.