For one Biden made a lot more concessions to the left and his campaign strategy included working with rather shutting out progressives. For another morgues were literally overflowing with Covid deaths, and enough people wanted something to be done about that to get Biden just over the edge. Hillary and Harris were both running campaigns strictly inferior to Biden’s, and not by a small amount, so given that Biden’s race was already very close you’d need zero additional information to predict their losses.
I think that one was more “we don’t take neoliberals”.
Then why did Biden win the next one?
For one Biden made a lot more concessions to the left and his campaign strategy included working with rather shutting out progressives. For another morgues were literally overflowing with Covid deaths, and enough people wanted something to be done about that to get Biden just over the edge. Hillary and Harris were both running campaigns strictly inferior to Biden’s, and not by a small amount, so given that Biden’s race was already very close you’d need zero additional information to predict their losses.