It is fashionable to blame the base. Instead, I suggest this underscores the genius of Democrats for self-destruction. Even when they do stuff for their base, they manage to do it in such an awful, miserable, grudging and painful way that the base haaaaaattttesssss it and everything about it.
Republicans are the polar opposite. They can totally and repeatedly fuck their base, but they do it in a way that keeps their base happy and wagging their tails. Some of this, of course, has to do with the utter and thorough purging of any Republican dissenters by said conservative base. But even setting that aside, Republicans routinely deliver diddly/squat and yet maintain an excited and engaged base.
Explaining why this is takes awhile, and the Democratic will to be stupid on this is astoundingly strong. I don't just mean among elected reps. I mean among mainstream Dems who are bitterly disappointed but deeply indoctrinated in Democratic Party philosophy. But here is a clue: You never hear Republicans (publicly) refer to their base as "retarded" or "the professional right" or "the angry right."