Under LaVell, BYU beat their rivals. LaVell, coaching staff and players KNEW this was a BIG game and seemed to be better prepared than Bronco and his coaching staff. The approach of it's just another game under Bronco hasn't been kind and thus the record of 3-6 against your rivals.
IF that was switched around, say 6-3 or 7-3 then MAJORITY of BYU fans would be throwing roses at Bronco instead of stones.