The only reason MMO's arent popular on consoles is because it's only now, that consoles are fully online. Or atleast, Its only now that the vast majority of the public are taking their consoles online.
PC is full of MMO's that are VERY popular, WOW, Everquest, Everquest 2, Final Fantasy 11, Star wars Galaxies, Guild wars. This is because the PC is more or less guaranteed to be hooked up with the net, with no complications.
If any of these games came now to Xbox 360 or PS3, I'm pretty sure they'd get played, a lot. Because I don't know of very many new consoles that arent connected to the net. Apart from those that decided to keep the firmware mod.
Everything is moving online now, RPGs, Shooters, Sports. And theres room for them all.