My best guess is because Dems alienated their power base. They used to be "the progressive choice", but over the years have shown they're just the same as pre-Trump Republicans, just a little less homophobic and racist. As long as I can remember, Dems usually struggle to rally behind a single candidate and it has hurt them in elections. There'd be, like, 3 Dem choices while Republicans get it boiled down pretty quick, and the Dems always betray the most popular to select the most status quo.

So, if I had to guess why Genocide Joe is still in and backed by them, it's because they recognize they simply can't afford the arguments and division. Nobody likes them except deluded libs. Nobody believes or trusts them. Progressives are disillusioned, many realizing voting doesn't work, others desperately looking for an alternative. Meanwhile, Republicans don't have that issue. The Trump cult wants him back, and failing that they'll overwhelmingly select the closest to him, because to them voting is more about patriotism and owning the lib. They'll vote so they can say it's rigged if they lose.