They (the Biden admin) purposefully withheld polling data that showed he had 0 chance to win, him and Kamala. Rather than attempt to find a candidate who would be popular in the general, they canceled the primaries.
Biden admin all but promised they would run 1 term in 2020, it was strictly an anti trump ticket in a post-covid world that was sick of him.
Trump was very simply a more popular candidate in 2024 who ran a much better campaign that wasn't hamstrung by NOT talking about Palestine, the biggest news story at the time. People entirely forgot how bad he ratfucked us on covid, they remembered the name on the checks.
Any facist who follows that playbook within the general election will do the same, especially if there's a opposition that quite literally knew they would lose and ran anyway. American elections are a joke because the American populace is fully captured by advertising and thus remain uneducated and ignorant.
Kamala could have won if Biden left later. Her best performance was immediately after he stepped down because they kept her from talking the entire Biden campaign (she was very unpopular). The second she had time to talk it was over, Kamala is a bad public speaker and terrible on the spot.
I'll even tell you the moment she lost it. When she said the wall was a good idea on national television to Anderson Cooper. That was it. American elections are a popularity contest, we (the American populace) don't take them seriously.