You know registered Democratic voters pick the nominee, not the DNC, correct?
If you want someone like Bernie to win, you have to fight with the establishment over the Democratic voterbase, and the Bernie campaign never succeeded in winning over the majority of the Democratic voterbase. You can argue this is because the majority of the endorsements and media were on Hillary’s side, but that was inevitable, you can’t just expect your internal Party opponent to roll over, you have to build up an opposing powerbase within the party and media.
Seems like a lot of socialists and progressives got too disenchanted from Bernie losing to ever accept that, though.