That's because the Dem base has had it with leadership failing to meet the moment. Individually, Dems are trouncing repubs in special elections and anyone who shows some fight and passion (AOC, Bernie, Crockett, Mamdani, etc) rockets in popularity.

If those candidates are our so called "Leadership", there is trouble in the future greater than what we have today!

Bernie isn't a Democrat, outside of putting himself in the running for Democratic candidate during Presidential elections. So a fair slice of his popularity with Dems who are disenchanted with Dem leadership is probably because he's *not* connected to them.