When Eelco wrote his PhD thesis 20+ years ago, with the first version of nix, it was just an academic project. Now it's a lot more.
Although nix has a very good community, and plenty of contributor activity, there is way more code contributed than docs.
Additionally, very few people are technical writers in the nix community (there is a lot of expertise tho, but mostly on the code side). Therefore we get today's nonstructured docs.
Finally, RH has WAY more money than the NixOs foundation. RH sells stuff. Nix and NixOs are community projects, relying solely on donations. They probably have people whose job is to write docs.
The documentation exists. That's the best thing about it and it's a whole lot better than none. It has been worse too. Compared to Red Hat though, there's just no comparison.
I really like Nix so far (just started using it), but damn is the documentation ever shit tier.