I've been mulling over this most of the day. The mistake the Harris campaign made was to focus on Trump being a terrible person. He is mind you, but everyone knew he was a terrible person 8 years ago, and he got elected. Biden was able to win as "not Trump", but even that was a close call.
Harris needed to establish her specific vision for America; Trump gave her all the leverage she needed with his "I've got a concept of an idea" in the debate. From then on, she should have focused on specific policy initiatives, introducing every single one with "I've got more than just a concept of a plan." (She did have some, I know, but we didn't hear about them often enough. Problem is, discussing policy initiatives requires more than the 30-second soundbite the media has become conditioned into generating.) Trump, as usual, lied his ass off, but he told lies that people wanted to hear.
More basically, she needed to acknowledge people's frustration with the economy. OK, so the numbers are promising. That's all well and good, but that's like pointing to the Dow Jones--it doesn't reflect what individual families are working with. That gave Trump an inflection point that he hammered home to devastating effect. And of course, he tapped into the fear of the Other, whether immigrants, LGTBQ, and then spun that web wider to include anyone who opposed him.
Back when I was in grad school at Bama, I was a member of the Alabama Student Party, a group of independents trying to make inroads into the Machine controlled SGA government. We got John Merrill elected SGA president, and the bulk of the ASP membership was convinced that we were headed to the promised land. There was only one problem--most of them thought that since we were fighting the good fight, God would smile on our endeavors and victory would most assuredly follow as day follows night. Myself and a few others repeatedly warned that good intentions mean jack squat if you don't have a well-considered plan, but we were roundly ignored, and the following year saw the Machine pull off damn near a clean sweep.
I suspect the Democrats were in a similar spot--once they established Harris as the nominee, they figured everyone would see that Harris was a vast improvement over Trump (and for all her faults, I maintain that she was). The DNC spent a LOT of money on media, but not enough on the message (apologies to Marshall McLuhan).