To be fair he didn't really sell out.

During the DNC campaign Hillary constantly flipflopped, trying to take positions that were his, and paint them as hers all along. Make the two completely opposite candidates appear similar. This was effective because there were many people who felt she was the better "brand" and thought "If they have the "same policies", why go for the off-brand."

If Hillary was sincere in sniping Bernies platform, I don't blame any DNC voters because ultimately the point is to vote for the change you need. But she's not sincere. Hillary is a manipulative, lying, piece of shit.

She's already distanced herself from some issues she took up during the primaries. So what Bernie did was "endorse" her on the policies she lied about. If you watch his initial "endorsement" you see it's basically his stump speech. Trying to forge a stronger link with her and the policies she lied about endorsing so as she continues backing off and revealing she is in fact a bag-o-shit, voters are less susceptible to her tactics.

He couldn't change the DNC outcome at that point, so it was a strategic move. He ensures more voters are able to see her true colors, plus because he "complied" the DNC has helped him a bit in his next goal of becoming head of the budget committee.

TL;DR - It was a hollow endorsement, to undermine Hillary Clinton.