There's a tactical reason for this. "Moderate" functions in our political culture as a relative term -- someone is a moderate only if there are serious political forces to either side of them. If there's no political movement to the left of Obama, he automatically is considered a liberal and not a moderate. A visible left-wing primary challenge would make Obama count as more of a moderate, and there seem to be some advantages to that sort of thing.
There's also a larger strategic reason. In a system designed the way ours is, the legitimate way to create new political options beyond what the two parties currently offer is to take over one of the parties through primary challenges. That's what the Tea Party has done, and it's been quite successful. The counterproductive and potentially disastrous way is to try to start a third party. Party officials must do their part to make sure that people on the left understand that working within Democratic Party structures is the best option. They should make it clear that primary challenges are the acceptable path, while third parties aren't.