The short answer to your question is that the Democratic establishment in general and Harris in particular repeatedly lied throughout the Biden administration, culminating in the bald-faced lie that Joe Biden was completely competent. This was done with the attitude of “well what are you going to do? Vote for the other team? Don’t be ridiculous.” There were so, so many other things throughout the Biden administration, it felt (feels) like a race to the bottom.
So Trump, who is notorious for lying, won. To be fair to Republicans, Trumps lies are more like crazy exaggerations sprinkled with outright bullshit which somehow is more palatable than being gaslit.
If the defense of the Democrats is “Well look at how bad Trump is!” it should at least be acknowledged that is one of the worst defenses possible. And in general, if my options are to be stabbed by person A twice, or by person B once but person B expects me to be grateful, I might just go with person A.
The end result is we will keep toggling between the two parties until one of them decides to run using sane people. I sincerely hope that will be the Democrats this year.