poodlestrike referenced the relevant twitter thread on this but imagine that your whole life every person of authority you've ever known is a Democrat, every meaningfully positive change in your life has come from Democrats, and you've been dutifully casting your ballot for the Democrats every 2-4 years even if you're in a deep red state and know on some level your vote doesn't really matter, but you just gotta keep your head up and keep trying anyway.
And then this guy from up in Vermont hits the scene like "yeah the Democrats ain't shit, I'm the only one who can bring about change, anyway I could have tried getting Jim Clyburn's endorsement but fuck that guy, anyone who doesn't already agree with me 100% is crooked, did I mention I have Joe Rogan's support? Anyway check me out I'm doing a town hall on Fox News tonight, and then I'm skipping out on the Bloody Sunday memorial to pursue my stupid strategy of running up the score in the most liberal state in the country so I can overrule the will of the majority elsewhere, P.S. Obama did a bad job in Mississippi and only I can flip it, but if I lose there it's because it's a red state"
Like look, I'm not saying that's going to be a shared experience or perception, but a lot of people do internalize the Democratic label as a part of their identity, and the people who do so are likelier to vote in Democratic primaries. If Bernie is jockeying for the Democratic nomination he should probably try winning those people instead of counting on a surge of unaffiliated, young voters (the least reliable demographic to turn out) to carry him across the finish line. Especially since in this instance all he has to do is pick up the label. If it's so unimportant, why can't he just do it, then?