Look at the vote totals, (as of right now) Harris 66 mi vs Trump's 71, compare this to 2020, 81 Biden vs 74 Trump.There is also this Pew article which shows the breakdown of non-voters
The simple conclusion is that the singularly most important thing a politician can do is excite and move their base. For how awful/low energy his campaign was this year, trump still excites and moves a base, while kamala was confused about what base to cater to- first calling Republicans weird then copying their platform and getting the dick Cheney approval.
This all could have been avoided if the Dems actually had a primary this year, but I think the DNC is actually afraid of progressives taking the party back over