I think people just (understandably) have little patience for new DSLs in general.
Another thing that I'm realizing is awesome about Nix as I'm getting my feet wet with CUE (also an interesting configuration language) is that in a way, the power of Nix makes it easier to learn than it otherwise might be.
So much of what we actually do with Nix leverages functions and libraries that are written in Nixlang, stored in the same monorepo as all of our build recipes. This means that once you get more familiar with Nix-the-language from using it in your own configs, exploring the implementations of the tools you use in building your packages and environment becomes easier and more natural. And Nixpkgs is then an awesome one-stop-shop for countless examples, and even kind of a quick reference for how a bazillion different pieces of software can be configured.
Maybe it's an acquired taste, idk. But I feel like the language is great at being simple when it can and surprisingly flexible when it needs to be. And Nixpkgs becomes a great resource for users, once they learn some of that relatively simple language.