We tried to be friendly with Texas but we were wanting to do it with all the Big 12 schools. The big problem is that they play a 9 game conference schedule and most of the teams don't want to play a team as good as BYU OOC.
There are basically two types of scheduling in the Big 12. Oklahoma and Texas tend to play one tough game a year -- BYU isn't "tough" enough for them; they are wanting to play teams like Ohio State (both have series with them), Notre Dame (both just played Notre Dame), USC (on Texas future schedule), etc.
The weaker teams don't want to play a team that tough. Baylor has played teams like Buffalo and UL-Monroe as their only non-Texas teams; TCU is playing teams like Virginia and Minnesota -- traditionally middle/weaker P5 teams. This is the typical thing -- one weak P5, one "rivalry" (SMU, Tulsa, Iowa, etc), and one FCS or bottom feeder MAC/Sunbelt team. BYU just doesn't fit in their scheduling philosophies.
There just aren't many opportunities to schedule Big 12 games OOC.