Beyond any criticism of the Harris campaign and the choices they made, it was always going to be an uphill battle. I think the image below (from March, but things didn't change after) really illustrates the state of the race. Other than Truman in 1948, no party has maintained control of the White House with an approval rating anywhere near as low as Biden had:
And if the last 8 years of consistently low approval ratings continues under Trump's 2nd term, I suspect we'll see a reaction against that too.