If for no other reason than they have an OOC rivalry with SMU already that goes back to the SWC days. With only two other non-conference games, they aren't going to want to use one of them to play BYU every year. My understanding is one game is to play a "regional" school (such as North Texas) and the other open to schedule various P5 schools.

And this is ignoring any "political" factors, such as what TCU and Patterson think about BYU.