He was popular among blacks, largely because he was Obama's Veep. Enthusiastic black voters gave Biden his key primary wins.
Sanders had white Bernie Bros and AOC socialists, Warren appealed to Hilary's Pumas, and Buttigieg had a niche following among progressive elitists. And nobody liked Harris.
But it's true that, apart from blacks, Democrats didn't actually like Biden. They merely hated Trump.
I don't know if he was "popular". Maybe he was respected. Or recognized. So they voted for him. But it did not necessarily mean he was popular. Other candidates had nothing compared to Biden who was Obama's Vice President so he had milked that for all it was worth.