This is my question as well for a lot of issues and I think part of the answer must be that the democratic party as a whole just isn’t that “left” when the rubber meets the road.
Another thing is that the places where democrats have overwhelming political majority is major cities more often than states. So you might get BLUE cities existing in just kinda blue states which exist in a country that is only half blue on a good day. The city, where the most political will to implement very left policies exists, is constrained in its actions by state and federal law and state and federal budgeting constraints which the city can’t effect directly.