It comes down to numbers, there is just to many mmos out there now and not enough mmo players. With every new mmo release it spreads the mmo player poulation out that little bit thinner. And you can only really play one mmo at a time properly.
Every mmo that is out there, and there is a LOT! Even the really old ones, you will always find people still playing it, even if its only a couple of thousand people. Over time this has just saturated the mmo market. Then you have world of warcraft which ive never personaly cared about as it looks like a very childish game, but this single game hogs a lot of the mmo player base.
Eventualy in another 2 or 3 decades all mmos will be using the free to play model as it will be the only way to make a profit on the game.
