MMOs are too hard to breach in to for most developers, WoW was a stand out hit which wasn't expected to be as popular as it is, its popularity has carried it years enabling it to grow masses of content and have a nice hefty budget.
MMO players expect a game that isn't up to 40 (if you're lucky) hours of content like most AAA games, they expect hundreds if not thousands of hours of content in the game, they get bored if they don't have that and quit. People quitting means the funds for the games development diminish, so it doesn't have the funds to grow the content fast enough to keep the rest of the players from quitting, so more quit and the cycle goes on.
The scale of an MMO is so mammoth barely any companies can afford to output the amount of content for its player base that it requires and it's just not sustainable. Think how much it costs to develop a normal AAA game that's just single player and has a story which lasts like 30-40 hours and think how much more content is required from a person wanting to sink hundreds of hours into your game.
The only business model I can see working for an MMO is F2P with a cash shop, to get the initial player base in high, and hope people buy stuff from the cash shop. People are less likely to quit if a game is free, because they're not losing anything by waiting for more content. A game being monthly payments and then changing to F2P doesn't work either, because they see the game as 'dying' so won't bother to sign up, so you won't get your numbers in.