I still have no idea how he actually managed to win the primary. None of the Democrats or Progressives I know in real life or online actually liked him. He was doing terribly and then all of the sudden he cinched the nomination in the span of two weeks. I sense there was a lot of back room power-broker deals going on and that was the real reason why so many of his competitors dropped out when they still had a good chance.
The Democratic party seems to be a lot more competent, and try a lot harder, when they are combating progressives, as opposed to when they are combating the other party.
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.
He won the nomination because people thought he could beat Trump. This turned out to be correct. Not many dems loved him (see poor rally attendance), but nobody really hated him either and so they went with the least-worst option (where the worst option is Trump winning again).