Alot of marginalized people were terrified of another term for the last doofus. They didn't want to rock the boat with anyone who was not center of the road and well established.
I feel like that would have been better reflected in Joe Biden's poll numbers in the races before South Carolina. He ultimately won, yes, and I don't doubt that plenty of people voted strategically but it genuinely seems like a switch was flipped when that South Carolina representative endorsed Joe Biden - his results became dramatically better after that.