Many of them didn't actually pay attention to Trump's actual platform - Trump voter remorse has been documented in the phenomenon of people trying to look up how to change their votes after the fact. They were convinced by rhetoric and propaganda, that Biden made gas prices go up and caused inflation, that the Democrats were sending Mexicans over the border to steal their jobs, etc, and by the typical populist appeals to rage and emotion, rather than reason.

This is because Americans, by and large, don't bother to know anything more about a candidate than what the memes on the internet tell them.