I think the phrase is, "It's the economy, stupid."

The economy had been trending upwards under Obama, and it peaked under Trump. If you're a Keynesian, you might gripe that Trump increased spending when the economy was doing well rather then saving for a rainy day. Then, the rainiest of all rainy days hit with the pandemic, which shot spending through the roof. That caused rapid inflation that became most noticeable after Biden came in. Most Americans either don't pay enough attention or attribute cause and effect to more or less random factors, so the experience is, Trump economy good, Biden economy bad.

Second, skepticism of the government is a facet of American culture, fed into from the national mythos regarding the Revolutionary War, by anticommunist propaganda about how the government doing stuff makes things worse, and also from experience with getting disillusioned from politicians not delivering on promises and the government generally not acting in people's best interests. Kamala comes across more as representing the political establishment, and her messaging doesn't tap into that dissatisfaction or contrarian nature.

Third, people feel like they're getting fucked, and Trump offers a clear, simple narrative of who is fucking them. And the narrative scapegoats people at the bottom of the social structure, who are least able to push back against said narratives, and who already have negative stereotypes about them. If you're not going to do that, then you either have to tell people they're not getting fucked, or you have to blame the people who are actually doing the fucking, who are at the top of the social structure, who are most able to push back against your narrative. Imo, in order to employ the latter strategy most successfully, you need a sense of solidarity, a sense that everyone is included in your movement and you won't allow anyone to be scapegoated or sacrifice anyone for your own advancement -and it's kind of hard to do that with the whole genocide thing going on.