The DNC is controlled by delegates elected by primary voters.

I’m afraid thats not entirely true. https://en.wikipedia.org/wiki/Superdelegate

Also, party leadership constantly changes the rules to suit the election cycle. They are a private entity and can run primaries however they want. In the DNC vs Sanders case, they successfully argued that they can elect canddates in a smoke filled back room if they so chose, and had no legal duty to fairness or in representing the will of the voters whatsoever.