I'm not sure what was so good about Wrath, but expansions since just didn't hit the mark for me.
They need to change something big to bring me back, same old stuff in a new place isn't enough anymore.
Wrath felt like the golden age of WoW to me, and I think a lot of other players.
Like neverender said, a lot of people who've been playing WoW since Vanilla are now 10 years older - will have a lot more responsibilities - and less time to commit to the game.
I remember spending so many ours grinding dungeons and raids in TBC and Wrath, but now I just do not have the time.