that relatively few P5 schools have (incl. our little sisters to the north). With the exception of a couple of blips (mainly the last 3 Crowton years), BYU has an an excellent last four decades, surpassed by only a handful of teams.
Why does anyone care about a long, sustained tradition of excellence like that one? When you sign contracts to schedule football games, you are often scheduling many years into the future, sometimes 10+ years out. If you are trying to make sure you schedule strong opponents, you are taking a risk if you schedule flash-in-the-pan johnny-come-latelys without a long history of excellence and consistency, even ones who have had a year or two (or a few years) of high achievement.
I think the donkey yewts have done well enough over the last decade plus that they are now viewed as P5 or near-P5 caliber, and will be seen even more so if they continue to win consistently. Their membership in the MWC hurts tem in this regard (and BYU's independence helps us), because if other conferences start treating them like a P5 team for scheduling purposes, how can they not give equal treatment to other MWC who finsh tied or ahead of Boise St in their conference?