From what I seen in the very early polls, not those for the Democratic primary, but those who matched up potential Democratic candidates against Trump late 2019 and early 2020, Biden was the best. I will admit, a lot of that was name recognition as Sanders was always second. Not doing as good as Biden did against Trump, but better than any of the others. If I remember correctly, Sanders won the first three contests prior to South Carolina, Iowa, New Hampshire and Nevada. None of which had any measurable black vote.
I think that scared the Democratic Party establishment; they can read polls as easy as I can. One Gallup poll showed only 47% of all Americans would consider voting for a socialist. The rest won’t even consider doing so. I would wager they imagined Republican political ads of Sanders himself stating from his own mouth he was a Democratic socialist went running through their heads. So perhaps the Democratic big wigs stepped in with Clyburn’s help to get Biden a win in South Carolina. It doesn’t take a rocket scientist to know how important the black vote is to the Democrats. Blacks make up roughly a third of those who affiliate or identify themselves with the Democratic Party.
https://news.gallup.com/poll/254120/less-half-vote-socialist-president.aspxBiden could also attract the independents voters with the others being much less successful. If the Democrats goal was to beat Trump, they chose the right candidate. Biden received 7 million votes from those who voted Republican down ballot. I don’t think any other candidate could have accomplished that. Now winning and governing are two different things. Biden has lost about half of those 7 million voters which is why the Republicans have taken a very slight lead in the generic congressional ballot. If he loses more, the midterms won’t be a happy occasion for the democrats.