People are saying it's because these groups aren't monolithic, but we're talking about a change from 2016 here, it's not as if they've become less monolithic over time. And besides, if it were confined to the more privileged subgroups, why wouldn't we also see an upswing in cishet white male etc support for Trump? That went down.

There are two reasons for this, that I can see.

First, Joe Biden is offering only platitudes, and this is in a time where neither party gains any ground by openly declaring their opposition to minority rights, except for trans people in particular (who I would guess did not swing toward Trump). If real liberation isn't on the table, and the rhetorical difference isn't that vast, then there are much weaker forces binding disadvantaged groups to the Democratic party. As to why now is different from 2016 in this respect, I'd just say it's sunk in more since we've been living under these circumstances for an extra 4 years.

Second, I think the right in America is staking out a position that's still incredibly bigoted and exclusionary, but on less clear identity-based grounds. People are being offered a chance to assimilate into the top end of a hierarchy that's based on "American-ness" rather than being quite as tied to intrinsic qualities. The Republican party is trying, successfully, to break any ties between Latino voters and the Latinos victimized by ICE, and to break the ties between trans people and the rest of the LGBT community. It's the same deal the right has always offered: you'll get screwed over, but at least you're still above those people. They just have to offer it to more people now that the identity demographics of the USA are shifting away from them.