The Lich King Fight, plain and simple.
Before that point in WoW's history (And outside of certain quests, ever sense), Resurrection has always been a game mechanic and never something that actually happens within the context of the character's existence. If you were dead, then that was it. While it's thematic for the fight and one could argue that Terenas, being a spirit and connected to Frostmorne, could resurrect those fighting against Arthas, it still takes away the aspect that Life and Death should MEAN something within the world. just having a revolving door where people could come back without issue just ruins any sense of suspense.