Well, DC Universe is on PC and PS3 so exceptions are out there.
However, the reason why you won't see MMOs on console as a general rule is because Microsoft (and to a lesser extent, Sony) place extremely strict controls on what goes in and out of their networks.
Microsoft are very much "our way or the highway" regarding games that use Live! and developers would have to relinquish control (and probably pay lots of money) to Microsoft. Cryptic Studios were writing Champions Online to be an Xbox 360 title as well, but they said it's impossible because of Microsoft's business model, not because of hardware limitations or anything else.
Secondly, as has been said, the interface for most MMOs only works with mouse and keyboard; I would dread to think what it would be like trying to play WoW with a joypad.