I agree with you that the Nix language is a problem, and most likely a mistake in hindsight. Even for someone with lots of functional programming experience it's ugly with bad usability. I'm not familiar with Guix, but I am with Nix and
> Guix is an advanced distribution of the GNU operating system
This sounds like it competes (?) with NixOS, but not with Nix. A reason for the popularity of Nix is that it's not only an operating system. Most Nix users probably haven't touched NixOS, but they use Nix as a package manager, build system, development environment, etc on their local machine. That target audience is a lot larger than those who want to adopt a whole new operating system.
This adoption of Nix outside of the operating system context naturally leads to more adoption of NixOS. If you already work with Nix anyway, it's not that huge of a jump towards adopting NixOS.
> Guix is an advanced distribution of the GNU operating system
This sounds like it competes (?) with NixOS, but not with Nix. A reason for the popularity of Nix is that it's not only an operating system. Most Nix users probably haven't touched NixOS, but they use Nix as a package manager, build system, development environment, etc on their local machine. That target audience is a lot larger than those who want to adopt a whole new operating system.
This adoption of Nix outside of the operating system context naturally leads to more adoption of NixOS. If you already work with Nix anyway, it's not that huge of a jump towards adopting NixOS.