When you run on the status quo, and the status quo sucks, people are gonna turn to whoever manages to present themselves as an alternative.
When you run to the right, and the people who like right-wing policies already have a party giving them the policies they want, they're not gonna switch parties, and you're just going to alienate the parts of your base/coalition that are affected by those policies.
Not an effortpost but I think those are two simple, straightforward responses to anyone being like, "How could this possibly happen?"
I'm also running around countering any "she lost because she's a woman" takes with with Tammy Baldwin in WI and Elissa Slotkin in MI winning despite their states going to Trump, which are two invaluable rhetorical data points, imo.