Well, it was quite clear that DNC turned against Bernie from the start of the campaign for the primaries.

Bernie almost won.

In some ways the DNC presented a larger challenge to Bernie than the RNC did for Trump.

From what I understand (which is based on Clay Shirky's Twitter feed, make of that what you will) Bernie had zero appeal to black voters, who are an essential demographic for the Democrats. He was very popular in a young, white bubble, which is also important, but still a bubble.