Just looked it up. There are still thousands of end zone seats available.
Looks like the problem is that BYU jacked up the price for the Utah game to $70-85 per seat, for the cheapest seats. Plus the weeknight, late start time.
The USC game has cheapest seats on sale for $35-40 per seat.
Maybe BYU miscalculated when they jacked up the price for the Utah game?