Because everything dies in this game. Everything is too trivial so there's no sense of danger.
There is not a soul alive who did not run from the Butcher in the original Diablo. He was as scary as he was vicious. A room fool of corpses and appendages everywhere. The stuff of nightmares. If you managed to run from him you knew he was somewhere in the dungeon.
There is nothing like that in WoW. No monsters, no threat = no monster just loot pinatas.
Discuss.