Acting like this is a brand new discussion that you've never seen discussed. I find it hard to believe you haven't seen this discussed ad nauseum. But in case you are serious here are the prevailing theories:
1) Bad luck due to remote geography
2) Bigoted school presidents of the liberal colleges in the PAC12 that didn't want to be associated with the Mormons
3) Truly concerned school presidents of liberal colleges in the PAC12 who were worried about BYU's "lack of academic freedom" and no med school
4) Big 12 officials that were worried about BYU's Sunday play stance
5) Big 12 officials preferred to expand east
6) BYU was too arrogant and pushy in negations with the Big 12
7) BYU preferred independence and BYUtv
Bottom line: no one knows