"They don't win any votes by supporting legalization (they have all those reluctant votes anyways), and publicly supporting legalization gives their far-right opposing party free ammunition (the DNC wants your kids to be stoned, urban crime, &c.). So it's all downside, with no upside."In today's polarized climate, you can say that about almost any issue whatsoever.
Compare the Democrats' reluctance to take on and lead on issues to the modus operandi of the Republicans, who have not hesitated to innovate and lead in the political sphere even when the ideas they float are laughed at and villified by their opposition and the mainstream media. Despite that they still manage to fire up their base, change the national conversation, and get their leaders elected.
Democrats don't have to have the equivalent of FOX News or conservative talk radio (they tried that with Air America[1] and failed), but they could show some backbone and actually lead rather than always reacting to Republicans framing the issues.
The nation has clearly moved on past believing in Reefer Madness style anti-cannabis propaganda, and even the risk-averse Democratic leadership should recognize that cannabis legalization is an issue with broad national support and that this is a leadership opportunity.
[1] - https://en.wikipedia.org/wiki/Air_America_(radio_network)