Altitude and religion, IMO, tide. Most of the PAC-12 teams with the exception of Arizona and Colorado don't play at significant altitude. Colorado hasn't been relevant since the early 90's, so they didn't gain squat on that level by moving to the PAC-12. Altitude doesn't help if you can't get the players. Utah on the other hand, being the center of LDS world gained a big recruiting advantage by going in the PAC-12. The west coast from Seattle to San Diego has a large population of Pacific Islanders, a majority of whom grew up in the Mormon Church. Mormon parents from that region wanting Junior to play P-5 college football and get a good LDS education are going to push for the kid to go Utah.
Also the Rocky Mt. states and the desert southwest are overrun with Mormons. BYU isn't a P-5 school. Where's a good Mormon kid who wants to play P-5 level football going to go? Utah. Throw in the altitude and the Ute's are a tough out at home.