Blame the at most 2 million or so antigenocide protestors, instead of the 75 million who voted for Trump. Considering Harris was short 8 million votes, it didn't really matter.
Blame Trump voters first, but also blame Harris for running a dog shit campaign where she leaned right to pickup Republicans, and ended up picking up zero R voters while completely alienating the progressives and losing 14 million votes from Biden's 2020 performance.
The Dems don't seem too upset though. They would rather lose to the republicans than let progressives have any sort of power.