Bernie knows which side his bread is buttered on. He didn't get multiple homes, including lakefront property by going against "The Party", nor against The Clintons (see: long list of dead people who did).
Bernie was NEVER expected to be a serious candidate. He volunteered to be a sparring partner for Clinton. Ever wonder why there was such a small number of Democrats running versus the 17 person Republican field? No one expected anyone other than Clinton to win the nomination.
If Clinton ran unopposed for the nomination, it would have given her no debates -- no TV time to air her
talking points platform. Plus, the underdog always get a grass roots base the others can't attract. Notice how often she tried to play the victim in all the investigations and as the topic of Republican's "attacks."
With Clinton running, there was going to be a lack of contributions for other candidates, too. She spent $1.2 Billion on the campaign. No money would have been available for other candidates. I think Bernie got most of his money form individual donors and IRS matching funds.
It wasn't until the mask started to crack on Clinton that Bernie began to gain popularity. He had a good chance of winning the nomination, which the DNC could not allow.
Bernie was never a Democrat candidate. He was a willing sacrificial lamb who received compensation for running and losing.
In another year, with another field, Bernie may have been the nominee. This was the Queen's year to be crowned. Screw what the little people wanted!