I think the real answer is all of the above. Biden waited too long to drop out, which didn’t give enough time to properly hold a primary. This resulted in Harris being nominated with no way to gauge how popular she would be. She then ran a terrible campaign spending too much time courting moderate Republicans. This resulted in progressives being disillusioned and not voting. Her stance on Israel and Gaza turned away Arab voters who also didn’t vote. All of this combined together to pave the way for a second Trump presidency.
It’s such an alien thought process that I don’t even know where I would begin with discussing politics with such a person.