For years after his campaign, he was the most popular politician in the US. He is still more popular than any politician who's active on the national stage. Above both Biden and Trump.
Clinton more or less identified the issue with him: No one likes him, he doesn't want to work with anybody. "Anybody" meaning the money-men who are determined the wreck the country. And so, he's unacceptable.
Even MAGA folks at least respect Bernie. I got that a lot in 2016. They knew I wasn’t a Republican, which made them suspicious of me until they found out I supported Bernie, at which point they’d actually say something nice about him.
I believe it. I know a few former coworkers who felt the same. They were all hard Trump by 2020 though, so that demographic I think is mostly gone by now.