1. You can run NixOS with Lix (I do)

2. Because Guix is less mature and more, ehm, opinionated, than *ix + nixpkgs

Guix users can just add the Fearware Guix channel which is just the nonfree firmware and blobbed vanilla Linux kernel, no propietary software it's added, just the necessary firmware to run your hardware (3D Radeon support, Nouveau+reclocking, some audio chipsets, and most of non-ant9k WiFi cards). That's it.