African-American vote
Biden owes his victory in large part to the great number of African-Americans who turned out for him on Saturday. He gained over 60% of that electorate, according to the Washington Post.
At the debate before the primaries, Biden touted his huge support among African-Americans in South Carolina, saying he was counting on their support.
Why do South Carolinian blacks love Biden? That in itself is a topic that requires further probing but what matters is they do and Biden can be grateful.