No need for them?

Genetic drift.

No actual direction to why they lost the crests, they just eventually lost them in favor of other things, though some may argue the crests are still present even in Tyrannosaurus albeit just reduced.

As far as I know, the large prominent crests were mainly traits of the proceratosaurids, which most likely didn't evolve into the later tyrannosaurids.

That is an interesting question. Maybe its bc its arms soon became display like carno. and wasn't there like a preserved dewlap on a tarbo. this could mean they had air sacks

They lost the Ancient Rizz Codex which contained all knowledge regarding crest shapes