The dems have a very large problem among women voters, who are the largest demographic of the electorate.
It is no secret that married women broke over 55% for Trump. Married women see things differently than single women.
It's also well known that single women between 18 and 50 tend to view the government as a replacement provider (Obama's Julia campaign) and it certainly doesn't hurt when the candidate is a young handsome Black man who is viewed as urbane and hip.
While single women have the numbers to impact an election, there is an enormous gap between older single women (2nd wave feminists) and younger single women (3rd wave feminists). Hillary made this gap 1000 times worse, by being dismissive of the young third wavers. Older feminists feel that younger ones didn't do the work and should just shut up and get on board. Older single women also have lost their sexual power over men and it makes them angry and resentful of the younger ones. Younger single feminists still command male attention and see older women as has-beens. This is a female rivalry that is epoxied into women's circuit boards.
While Bernie has a strange hold over the young white male Bernie-bots, he is a turn-off to women in general and particularly young feminists.
Elizabeth Warren has largely the same problem Hillary did; her appeal to 2nd wavers puts off the 3rd wavers.
And Joe Biden...hoo-boy. Feminists will tolerate Slick Willie getting his cigar polished in the Whitehouse by a fawning adult woman, but they aren't going to excuse Creepy Joe pawing at little girls.
To unite younger and older single women, dems need a good-looking, younger, hip man -- and he can't be too white or he'll anger the intersectional crowd who are pushing dems hard left.