Hillary had the DNC pull dirty tricks on Bernie and the supposedly neutral DNC kept plotting ways to cheat him. This was all revealed to be true in Wikileaks release of DNC emails. It was so embarrassing it forced the Chair of the DNC to resign. Later the DNC defended itself in a class action suit by arguing that it was under no legal obligation to be fair to candidates.
The real question is why Hillary and the DNC went over the line to undermine Bernie. She had enough superdelegates in the bag to win the nomination that she would have won even if tied with Bernie for the elected delegates.
By the way, I am not a Bernie Bro and I did not vote for him. But the man was cheated. Would he have won without the DNC thumb on the scale? We’ll never know.