The dems are never going to pass voting reform for the same reason the UK labour party (a considerably further left party than the dems) has never passed it despite pretending they would consider it for multiple decades now. They benefit from FPTP. All they would be doing is diluting their power and handing over a huge portion of the political landscape to socialists who would immediately become relevant, they would then be forced to actually come to agreements with those socialists as opposed to just completely and totally ignoring them as they do currently.