BYU thrashed Utah in the 70s, 80s, and early 90s. They were pretty even in the late 90s and 2000s. Utah has dominated in the 2010s.
Most Utah fans (ALUFs especially) are still reacting to the trauma they experienced in their youth. They /finally/ know what it's like to dominate their rival and they delight in tormenting their tormentors. Kind of like when the little brother finally realizes he can take down his older brother and does so with too much gusto.
BYU's time will come around again.
As will Utah's.