Marijuana legalization doesn't actually poll that well among core Democratic constituencies. IIRC there's something like a 20-point gender gap in support, and in - from the Dems perspective - the wrong direction.
I didn't say Democrats should run on the issue. Just get it on the ballot and get people out to vote for it. It can be a great way to get young people out to vote, especially young males.
Also, women range from soft supporters to soft opponents of Marijuana legalization. If anything, more Democratic politicians endorsing it and making it seem mainstream will change minds. Whenever Marijuana legalization does well, it's because women voted for it or at least didn't oppose it in large numbers.