Making an MMO can be hard. You can put years into developing not just the game, but the necessary infrastructure, only to launch it and have it die within weeks. Sure, Blizzard know how to do it, but few other developers have managed. Microsoft certainly tried, and tried twice (Marvel Universe, True Fantasy). They also failed twice, with both projects canned, something Shane Kim takes full responsibility for:

I'll admit MGS has not had success in the MMO space, and that happened under my leadership, so I take full responsibility. It's a tough and challenging space that's evolving all the time – a lot of shifting sands. We haven't been able to crack the code.

There there Shane, chin up! What do we do when we fall off the horse? Shane Kim Talks 360 MMO Washouts [Edge]