This has been posted about several times over the past few years but it's typically been written off rather quickly because it's not news fans like to hear.
The rumor back in 2011/2012 was the "smaller schools" in the big 12 who are obviously much more vulnerable during realignment were very upset when BYU did not join their conference to help stabilize things during a time of need. According to those saying this years ago... BYU backed away from the negotiating table when OU and UT starting flirting with the PAC. This made the more vulnerable schools very upset. OU and UT allegedly really do like BYU, it's the others that don't.
Have you guys not noticed the lack of ANY scheduling agreements with the Big 12 since late 2010/2011? Did you assume the league hasn't scheduled BYU because they love BYU?