Because the Democrats tried to run Biden again, despite the obvious-to-everybody signs of decline and unfitness. Then, when that became impossible to ignore, they anointed Harris. (Thereby overturning the results of the primaries, which created bad memories from the previous two campaigns.) Then Harris said that she wouldn't do anything different from Biden, despite people being tired of Biden.
And because the electorate had kind of forgotten what Trump was like, because they'd just spent four years seeing what Biden was like. There was a bunch of stuff that Biden (or at least his people) did that didn't really resonate with voters, and a bunch of them voted for "not that".
The other thing they did wrong was, they were a year late in prosecuting Trump. Trump managed to delay things out to the point that the campaign (and then the office) protected him. I don't know if Democrats delayed deliberately, so that the prosecutions would be damaging Trump as the campaign season started, but if so, they were well-paid for that bit of attempted chicanery.
And because the electorate had kind of forgotten what Trump was like, because they'd just spent four years seeing what Biden was like. There was a bunch of stuff that Biden (or at least his people) did that didn't really resonate with voters, and a bunch of them voted for "not that".
The other thing they did wrong was, they were a year late in prosecuting Trump. Trump managed to delay things out to the point that the campaign (and then the office) protected him. I don't know if Democrats delayed deliberately, so that the prosecutions would be damaging Trump as the campaign season started, but if so, they were well-paid for that bit of attempted chicanery.