Joe Biden had a simple theory of the case: Twitter was not the Democratic Party, let alone America. Moderates, many of them elderly, were a majority of the party, if not the activists. If he united them, he would win.
It was clear even after Iowa that he was right; the real question is whether he had enough support, and could perform well enough, to be the last man standing in the moderate lane. Things were looking grim after Nevada. However, black voters in South Carolina saved his bacon. Mayor Pete, Klobuchar, and Bloomberg dropped out within a matter of days after learning that they didn’t have meaningful support in the minority community, leaving Biden in a one-on-one battle with Sanders. After that, everything fell into place, and the rest of the campaign was a formality, particularly when the pandemic intruded.