what all but 2 teams said? Thats right. BYU. The biggest reason is that BYU was a bigger and better program.
It is the same everywhere. Alabama has several SEC Conference rivals. Tennessee, Auburn, and LSU are all Alabama rivals. How many rivals does Vanderbilt or Kentucky have (in football)?
BYU was the Alabama of the WAC. Even in the MWC no one had more conference championships than BYU (despite the crowton years).
Other things doubtless factor in, but most of it is because BYU is (usually) a good football program and is close by.