I think they are scared little pussies who are afraid that if they let the left dominate the party, it will be a slippery slope to communism, and the American economy will collapse. The democrats fear change.
Despite clearly Social Democracy like Bernie or AOC is nothing of the sort. Denmark is mostly Social Democrat, and so are the other Scandinavian countries, and the freedom here is way above USA, and the wealth above what most comparable countries have achieved.

Now they got the Republicans doing in ½ a year, exactly what they feared the left might have done in 20.