One is that that the Woke "systems" for race and "gender" come from completely different, incompatible "academic" theorising. One's from "queer theory", which tries to blur all boundaries, and the other is from "critical race theory", which insists boundaries are sacrosanct, along with some sort of concept of original sin.
If you applied queer theory to race, then it would be your duty to "queer" racial boundaries.
If you applied critical race theory to sex, you could argue about the historical oppression of women, and how men couldn't just opt in to that history of oppression.
And these things can be different, because they're just pseudo-academic waffle with no real attachment to reality. There's no attempt to make a "universal model" with one set of rules. It's different rules applied by different people.
Some poor philosopher got in trouble a couple of years ago for trying to figure this out - she tried to reconcile them by saying "why not?" to racial identity. This managed to upset both Woke sides; the "trans" side were upset at the suggestion that their identity was in any way not innate - obviously it was a real thing, unlike any other choose-you-own-identity claim. And the "race" side were upset at her suggestion that racial identity could be a thing.
You can look the Wikipedia article on that, although I don't think learn much to answer your original question, but you can admire the histrionics.
en.wikipedia.org/wiki/Hypatia_transracialism_controversy
The best I've seen them do as an argument is that you can look at a whole community, and they'll be of one race, but every community has males and females. Therefore you can identify a separate disadvantaged black community, but you can't identity a separate disadvantaged female community. Every man has a woman in his family, therefore the female identity and any oppression is part of his culture (inherited from his mum!), unlike the identity of a different race.
Something like that seems to be where they end up if you keep pushing.
In practice, people particularly in America will use the "self-id" principle for race, if they don't care to call you out. Maya Forstater's employer in her tribunal did say that they would accept Rachel Dolezal as black, because they use self ID as policy. Individual actors, including companies, will not dare to challenge a race claim.
But the mob will challenge it,just like they would challenge any "cultural appropriation claim".
As a previous poster has said though, there is probably a more fundamental reason why the two have ended up different - "queer theory" was invented by and pushed by boundary-breaking men. It's a rationalisation of what men want - no boundaries. And the "critical race theory" thing is a rationalisation of what a particular type of activist wants - continued racial grievance with clear boundaries to keep themselves in work. They're both constructed to achieve activist goals, not actually neutral academic studies, and they have different goals.
("Constructed to" might be a bit strong, but certainly they've evolutionary adapted to fill those roles, and it's clear that they attract proponents of those goals.)