There were many, many reasons. The two biggest are inflation and the border. I can defend the Biden administration on inflation, as it stemmed almost exclusively from a black swan event in Covid. Whenever you pour money quickly into an economy as we did, and needed to do, to combat Covid, there was always going to be a downside of doing so, One of those downsides was inflation. But Team Biden doesn't get off Scott free with this with me, as they were very slow to allow the California ports to operate 24/7. But the money that poured out, and in some cases enabled people to get paid MORE to sit home than attend work, started under Trump.  
.
The thing that is impossible to defend is Joe's inept control of the souther border. I mean it was so bad, you had Democratic mayors in blue cities complaining about it. They bungled this issue 8 ways to Sunday. I give them no cover here as it was a catastrophic failure.  
.
Like I said the other day, there was a general feeling of unease watching the incredible horror play out on our TV screens every single night re: Ukraine and Gaza. They happened on Biden/Harris watch and, fairly or unfairly, they will get labeled with some of the blame.  
.
In hindsight, Joe not stepping down after year 3 was a disaster. He was buoyed by the fact that the '22 midterms were not the red wave so many predicted and I think that influenced his decision to stay. His delayed exit from the race gave Harris much too short a time frame to do all the necessary things there is to do to run a successful campaign. It also didn't allow for time for us to have a primary where a field of Democrats could have made their case for the highest office in the land.  
.
The disastrous debate performance from Joe caused so many problems. He then immediately endorsed Kamala which essentially cut off any other candidacies. I don't think an open primary would have helped us at all as that would have lead to a totally fractured Democratic Party with no time to heal before the election. Many people didn't trust the DNC, Joe's wife/family, Kamala, staffers, etc for hiding Joe's mental struggles from the public so when shit hit the fan at the debate, our side was in deep trouble for not just lying by omission, but by giving Kamala far too few months for her to prepare and get up to speed to battle Trump.
.
People wanted answers to hard questions such as what would she do to bring down costs and secure the border, but she spent most of her energy telling the public Trump was a threat to democracy and focusing on abortion. Important issues? Yep. As important as inflation and the border? Nope.  
.
Joe's approval rating was typically in the high 30s, to very low 40s. 70% of the peeps said we were on the wrong track. He was NOT popular.  That being the case, it was evident it would be a "change" election. Running the VP from an unpopular administration for Prez is not change. Kamala further made this worse as when she appeared on The View and was asked what she would do differently in hindsight, said nothing really. Ouch. BAD answer. Especially when your motto is "Harris/Walz: A New Way Forward."
.
Trump surviving the assassination attempt with that now iconic photo of him bleeding, fist in the air with the U.S. flag just over his head is right out of Hollywood. I am sure circumstances from that day's near tragic event persuaded some voters at that moment to vote for Trump.
.
I am sure there was some racism and some misogyny mixed in to his votes, but most of those people were voting for him anyway.
.
I think Democrats were on the wrong side on some cultural issues which hurt. From "Defund the Police" to "Men can have babies" from "there are 85 genders" to "it's fine for bio males to compete in women's sports and use women restrooms", we got our clock cleaned. Democrats crushed Rs on cultural issues such as abortion and gay marriage, and I think we let our hubris get the best of us and think we could win all these other culture wars and that fizzled. The mainstream Dem party needs to stand up at times to push back against lunacy from the far left, and when we did manage to do so, it was usually far too late for it not to tar the whole party with bizarre concepts and ideas.  
.
We need a better way to talk to men. That is clear. There are so many disaffected younger men that are lost, that should be Dem voters that we threw into Trump's hands. My party used to kill it with the working class but think about this...She won with voters making north of $100K...but HE won with voters under it! It is astonishing how quickly things have changed and I think we took these voters for granted. We need to start reaching back out to them asap.

Stimulus funding that caused inflation was a result of now known to be unnecessary lockdowns that crippled economic output.  So it was one fuck-up to counteract another fuck-up.

Even white Americans only went for Trump 57%.  So Native Americans going for Trump by 65% makes them the race that preferred Trump the most.  I wonder why.