Not having enough support is the only reason he lost. The DNC didn't go in and tamper votes.
I've seen many sanders supporters get upset over things that they perceive as unfair like how candidates dropped out to stop splitting the vote.
Every single candidate has had complaints about being treated unfairly. People were calling joe's campaign dead in the water. Yang got upset enough that he called out networks in a very public manner.
And yea, Sanders wasn't supported by the establishment. But like we've seen with Trump, the establishment doesn't matter if the candidate has real support. Obama was a no name senator that beat the most establishment character ever because he had that support.
Sanders had a lot of problems as a candidate, and those are the real reasons that he lost rather than conspiracy theories about machinations by the elite.