Corporatist 1% hacks at the top of the party. Vile, violent stormtroopers on the streets, a whiny gaggle of SJW cry-bullies and weaklings demanding ALL the attention and a compliant money and status grubbing phalanx of media members who will say anything have pushed out actual progressives.
Not to mention the Dems screwing Bernie over in such a public and unapologetic way has really chapped the hides of these people.
I bet if they really wanted to feel rage towards what the Dems have become they would look back on Obama and realize that in eight years he didn't prosecute a single banker for the looting they did during the financial crisis. Instead he cozied up to them. Use a gun to rob a bank of a couple of grand and go to jail for ten years. Run a bank and steal billions and the President will come stay at your house for the weekend.
And aside from the ACA, what did he and the Dems do that actually helped their nominal constituents? Despite that some people got insurance and it helped them, many others got screwed. And really the ACA was not for the little guys, it was designed to benefit some other folks.
I can't think of anything else that Obama and his people did that actually helped the worst-off, the marginalized, and the weak that they claim to care about.
So why would these actual progressives stay? With any luck the top donkey-men will not realize care what's happening until it's too late.