The armchair experts always crack me up. We all want simple answers to extraordinarily complex situations, like the social dynamics of elections. None of this is simple, or, probably even knowable at all. Both political parties are basically mashing buttons, hoping for the best. This year, Trump's button mashing apparently worked better than Harris's. (I think) that sucks. I'm very disappointed with the results. And like everyone else, I have no idea why the results are what they are. Maybe we can learn some of the reasons in the coming years.
One challenge Democrats face, I think, is the double standard situation we're now in. The cat's out of the bag, so to speak, and Republicans do, and are expected to, win at any costs. It's expected that they'll blatantly lie about things as inconsequential as crowd sizes, to immigrants eating pets, to the outcome of a free election. It's expected that they'll refuse to follow basic norms, even ones they made up (refuse to divest from personal investments, refuse to show tax returns, go ahead and appoint supreme court justices in final year of presidential term, which they claimed was not allowed for the previous Democratic president). Republicans have become the quintessential examples of demagogues the founding fathers worried so much about.
Democrats, for all their many problems, do basically operate in good faith, I believe. They do have a floor to what they're willing to do. When a Democrat behaves unethically, usually they're asked to step down by their peers (e.g., most Democratic Senators called on Bob Menendez to resign after his whole gold bar fiasco. That would just never happen in today's Republican Party). So the Democrats really are fighting at a colossal disadvantage. Republicans are playing Streetball rules against Democrats Basketball rules. It's just not a fair game.