What does ND game by losing BYU? Not much, just add in another team who wants to play them and many would do 2x1's.
What does BYU? A lot. Big brand name team on national TV, ND may or may not be good but get a BYU win gives some credibility to the schedule (they year is an outlier). Good for recruiting, fan base likes it etc etc.
BYU has much more to lose vs ND.