Texas fans are watching a lot of the other Big12 games and matchups because they want to see how the rest of their conference foes are doing. BYU is not in a conference so outside of BYU fans and the fans of the team we’re playing, we don’t have another automatic built in audience.
My Ute friends watch a ton of the other Pac12 games for this exact reason. It’s why conferences get such large TV deals.