An intersection of many things. There is this increasing narrative that Democrats do nothing for the working class, and this is a recent narrative since traditionally the working class has been the base of Democrat support. It is also pretty clear that Kamala Harris didn't resonate well with swing-voters. The issues of the economy and taxes loom large and this was better emphasized by Trump (or so it seems). And while Republicans are largely a united party on issues, the Democrats are more like a coalition, so people opt in/out to vote on issues (e.g., the middle east). There are probably other reasons as well, but one that remains massively puzzling (speaking as someone who finds Trump utterly repulsive and corrupt) is Trump's charisma and cult-like support among his base.
I will note with some surprise that KH didn't even win the popular vote. I anticipated it being closer (
@emir was correct about the popular vote).