Bronco was hired, at least in part, because he was willing to buy-in to what BYU wanted at the time (which was that there were more important things than success on the football field).

Not to take anything away from LaVell, but HC enforcement was very different in the 70s and 80s. The game wasn't as big as it is today and for many schools, the "win at all costs" philosophy hadn't yet caught hold.

Priorities began to change for the Church in the late 90s and LaVell got old and tired around that same time. Then, the Crowton-era rape scandal (and the terrible teams he fielded) sealed the deal. By the time BYU started looking for Crowton's replacement, the school was certain that football wouldn't be the #1 priority. They couldn't/wouldn't keep up with the CFB arms race (widow's mite and all that) and they certainly didn't want to chase on-field success if it was going to result in bad PR (like the rape case).