As an American, it's obvious why Harris lost
Lots of posters on this subreddit appear to be British. Here's my American perspective as someone not in the media class, who is a lot closer to the 'median American voter' than most you may hear from in the media that reaches Britain.
Everyone I knew here in Ohio is either a Republican or someone who wasn't going to vote for either. You have pockets of very blue voters (eg, Cuyahoga County) but most of our cities weren't decidedly blue.
The apathy was so apparent. When I went to vote, it was unusually empty–midterms got more turnout in the Greater Cincinnati area–and the Harris people were leaving because nobody wanted their pamphlets.
Every conversation leading up to the election signalled a deep sense of resignation; Harris was running as a lite Republican, letting the MAGA movement completely control the political narrative. If you were a genuine Democrat, you didn't want her. If you were a Republican, you wanted Trump anyway. If you were able to be swayed (the suburbs were filled with such people), the Democrats had thoroughly established that they were the party of doing nothing and promising nothing, while the GOP at the very least promised change.
The fact that she won 15-20 million less votes than Biden (while Trump got 3-4 million less than in 2020) is not surprising to me. The fact that she wasn't Trump wasn't able to undo (1) the toxic brand of the Democratic Party as incapable or duplicitous OR (2) the alienation of the Democratic coalition by her campaign (pushing away the white working class, Latinos, Middle Eastern-Amerians, young people, the environmentally conscious, etc).
If you were alienated by Harris but still identified as a Democrat, you likely voted for other Democrats but not Kamala Harris (eg, Sherrod Brown won more votes in Ohio than Harris). If you were alienated by both Harris and the party, you likely just didn't vote. Both groups nationwide added up to 20 million less votes for the Democratic ticket.