The real culprit is the Democratic leaders continually lying about Bidens degrading mental faculties and waiting until it was too late to effectively have a Primary. If they had pushed him out months earlier, they could have had a primary and put up a far better candidate than Kamala Harris. They waited to the last minute, when the money was already donated to the Biden/Harris campaign and the logistics for returning and then requesting the money again was considered too onerous by them.
It also did not help that Kamala was a very poor interviewer. The only subject she could talk emphatically about was Abortion. When it came to the issues of migrants, economy, foreign affairs, she was constantly avoiding the question or using word salad explanations that evidently turned many people off. I noticed a lot of people were complaining about her interview performances and speeches.
I firmly believe this was an election the Democrats could have won if they had put up an effective, articulate, concise, and charismatic candidate and I don't think it would have mattered what sex that candidate was. Kamala Harris was just a very weak candidate in my opinion.