I'm not the person you're asking, but I have a few thoughts.
The modern West, where the current concept of nonbinary is more prevalent, is marked by its individualism. Third gender labels are no longer imposed and are identities the individual can take for themselves.
I mentioned in a pp that people with nonbinary identities tend to have additional complicating factors that would make them feel out of place. In days gone by, communities were diverse. A village would be made up of you, your neighbour, little Polly with her skipping rope, that odd man in the pub, old Mrs Smith down the road, the lads at the coal mine. Everyone had different personalities, worldviews, jobs and hobbies.
Nowadays we have to choose our communities, and we largely do this by finding like-minded individuals. We gather around a certain idea or interest and sometimes, like some real-life Flanderization effect, it can become our defining feature.
As for sexism, I think sexist stereotypes have strengthened since the 80s or so. But life as a woman in the West is more favourable than in some of the aforementioned times and cultures. It is notable that women are more likely to identify as nonbinary, and I definitely think gender identity theory hinges on a desire to reject gender expectations, but to say it's only because of sexist culture would ignore all the sexist cultures that don't have this concept.
Personally I think the current Western concept of nonbinary arises from a culture that is both sexist and individualistic, and that's what makes it different from third genders in other places.