Many will say that it's because MMOs are so complex and rich that they cannot be managed with a console controller, and that's partly true -- imagine playing World of Warcraft with a DualShock -- but that's more of a design issue. A game meant for consoles, rather than one ported over from the PC, should have such limitations in mind. There's also the closed environment of the console, which is harder to plan patches for, and the still-in-its-infancy online service for the three major brands. On the bright side, there would be no need to worry about system specs; every player would be getting the same experience. And there could be added value in game achievements, which is a system that doesn't currently exist for PC MMOs.
Are there more reasons against console MMOs than there are for them? Should developers bother with the consoles at all?