Cataclysm came out and it was harder at first - so they lost 2 million customers out of frustration.
WoW "died" because Blizzard made the game harder after WOTLK.
They have since nerfed everything back to easy-mode and introduced PuG raiding via the LFR tool to try and bring back everyone who left out of Cataclysm frustration.
But nobody cares anymore because they want to play D3 or GW2 now. (Or whatever the next flavor of the month is)
MoP will be WOTLK 2.0 with Pokemon mini-game. Expect it to return to WOTLK easy-mode farming.