I see Biden as the virtual equivalent of "Generic Democrat" at his point. He was Obama's VP for 8 years and absolutely no one in the center left has been taking any significant shots (public) shots at him so far. Sure there are rumblings about his old Senate votes and the like but they aren't being aired yet outside of political junkie circles. Mostly he still is "Uncle Joe", and no one wants to cross him yet because it is possible he will decide not to run, so why get on his wrong side at this point. That will change if Biden enters the race. Right now though he still carries the all purpose label of "electable Democrat". And he is a valid contender, but his current lofty numbers are inflated so long as Biden floats above the fray.
"the black voters in the poll selected Biden by 43%, while Booker, Sanders and Harris were all tied at 13%." So of the "other candidates" that you speak of, or more specifically the ones who are polling in double digits along with Sanders, two ARE well known to the SC Democratic electorate, because that primary electorate has so many African American voters and Booker and Harris are our only two Black Democratic Senators. People may not know a whole lot of details about them but they know that. The fact that Sanders as a white candidate is tied for second with two black candidates, is not bad for him at this point while Biden is leading the pact untested in a competitive phase. The same is true of O'Rouke. All of his positive press last year came from him running against a Republican, not fellow Democrats.
Warren is holding her own with Sanders which is significant, but SC comes after Iowa and NH and those contests may establish stronger momentum for one of them over the other.
The way I see it is there are a whole lot of votes up for grabs, and any one of the current leaders in SC could well have hidden strength as the second or third choice of many people now leaning toward someone who may not prove to still be viable when SC rolls around.