The biggest difference between Stacey Abrams and Rev. Barber on one hand and Bernie on the other hand is that the former see all this as a civil rights struggle that will take many years of organizing and Bernie (or his stans moreso) thought they could win based on anger.
I'm not really saying this as something that is anti-Bernie. But this is rooted in differences between the Black experience of change in American history and the college-educated white experience of change.
It's telling that basically every third party campaign in American history has been extremely white while Jesse Jackson and AOC and Abrams and Barber, etc., have never really gone down this path.
Obviously, this is all far more complicated than what I am presenting, but one can only explain so much on Twitter.