the other choice is orders of magnitudes worse

Both can be true.

The other side being worse doesn't necessarily motivate your base to support you, you need to actually motivate them to get out and vote. It also doesn't necessarily motivate people on the fence either. If you aren't an attractive candidate, you can't rely on the unattractiveness of your competitor to win you the election.

It seemed the DNC banked on the public caring that Harris is a woman of color and popular among celebrities, and I doubt the public particularly cares about any of that. Her policies were weak and she came off as not really having a plan, or in other words, riding on Biden's coattails. That's not a compelling argument...