It is one of a couple things:
1) There are more R's than D's that voted in the election and Trump just turned them off, so they voted for Biden at the top of the ticket and then R's down ticket.
2) Some funny business went on.
They lost seats in the house that were supposedly safe, they lost state legislatures, and it was the prime opportunity to flip the Senate and that isn't going to happen. So, 1 or 2 happened, and honestly, neither are good for the D's.