You already knew that the DNC liked team-player, loyal soldier Hillary rather than independent, more-interested-in-protesting-than-governing Sanders.
You need to put emphasis on 'independent'. Sanders was not a member of the Democratic Party until not long before he started running for President. Why is there any expectation that the leadership of the Party? They are there to promote the interests of the Party, which includes promoting people with demonstrated commitment to the Party.
In exchange for public funding of the parties' primary presidential elections, the general public get some say in the presidential candidate that is selected. Then again, I live in a state where the Democratic Party selects its convention delegates by caucus, where only Party members (or people who state an alignment with the Party) participate in selecting the candidate.