Bad bowl and postseason access. If BYU would have stayed in the MWC, they could have had a chance at a NY6 every year even if not undefeated.
They still had a chance as an independent, but a lower chance (both because being undefeated was less likely, and the auto-access was non-existent).
But BYU independent regular season >>> a higher chance at NY6 game.
Big 12 will be much better post-season access, so get ready for more meaningful bowl games for BYU.
Of course, non-NY6 (and now non-playoff) post season games are pretty much all meaningless anymore anyway.