I don't entirely disagree that the incentives aren't there, but I'd say that the bigger problem (particularly over the past ten years) is that they've allowed the entire party platform to just be "we're not as bad as the other guys, that's good enough, right?" and they're insulated enough to believe that that's true. I think the "common wisdom" in the DNC is probably still that progressive policy is actually politically dangerous; that is to say, they don't know that they're popular, or rather they still think they can build a big enough tent to bring in the right-wing rather than expanding the voting bloc on the left.

Which leads the party members into "just get us back to brunch" behavior, and makes the structure which causes a lack of meaningful action into an effect, rather than a cause. I'm hopeful that the AOCs, the Mamdanis, and the Talaricos will show the deficiency of that structure, but I don't think it'll be quick.