Basically Sanders was winning because all the moderates were split between a few candidates (Biden among them). On the days leading up to "super Tuesday" when 4 big states had simultaneous primaries all the moderates besides Biden dropped and endorsed him and he was also endorsed by Jim Clyburn a major democratic player(also black).

The people who dropped and endorsed all got some position of power. So essentially, Biden played politics and won and many people don't like the politics he played.