The current Microsoft Game Studios Corporate Vice President, Shane Kim tell Edge in an interview why there's been zero MMOs on the Xbox so far.
Two anticipated titles have been canceled, Marvel Universe Online and True Fantasy Live Online. This is not stopping other companies, such as Sony however. Currently Sony is aiming to get DC Universe Online and The Agency finished and released on their Playstation 3.
So… why can't Microsoft keep up with the competition MMO-wise? Shane Kim explains (by being brutally honest):
"I'll admit [Microsoft Game Studios] has not had success in the MMO space, and that happened under my leadership, so I take full responsibility," – he said to Edge.
He also mentions that it’s a lot more expensive to develop an MMO and you must find the "right mix" – something that Microsoft hasn’t found yet.