I'm not going to reply to a particular post because I'm joining almost 300 posts in, but here are a few things:
People are saying Sanders is popular because he only criticizes / he isn't president / etc. In March 2009 Obama's approval rating was +34. In March 2001 George W Bush's approval rating was +29. Clinton was +38. Pay attention to facts, if he was president his approval rating would be higher. His approval ratings would be under +28 after six years of being president for sure. It would be extremely unremarkable for a president to be +28 in March of their first year (it might even be remarkably low). It's incredible that someone who isn't president is that well regarded.
People are saying Sanders supporters are lousy/annoying/racist/sexist. He's at fucking +28. Remember when Hillary Clinton got in trouble for saying a quarter of the country was racist*? You're saying 70%+ is without even counting the antisemites.
I saw a little bit of the implication that Sanders is somehow racist or at least that he is disliked by racialized groups because Clinton did so much better with black voters. You can look up approval ratings for Sanders with black democrats during the race - he was plenty popular. Black democrats** liked both Clinton and Sanders and just liked Clinton better. I know choosing between two good options is totally outside of the Overton Window for American politics, but that is really what happened. All of these things can be true: Black Democrats liked Bernie Sanders and his message; Black Democrats preferred Hillary Clinton to Bernie Sanders; Hillary Clinton beat Bernie Sanders for the nomination in large part because she was the preferred candidate of black Democrats. No contradiction there.
* She was right, but if we are engaging with that instead of making pithy comments - and thus acknowledging that most Americans promote racism - I'd point out that the radical left is the home of anti-racism activism, and also refer you to my third point.
** Remember, these are just dumb aggregates. People who are black and American and members of the Democratic Party are still people, not fungible pieces of a voting block.