Nix has been around for over 20 years, and a lot of nerds volunteered to make it better.

Nixos not being fhs compliant is a most likely also a big factor. Before you could emulate FHS you couldn't use language specific PMs like npm, and instead were forced to build those packages with nix. So now it's filled with packages that other distros wouldn't ever add. Even worse, I've seen python libraries have separate packages for different python minor versions.

The Arch USER Repository is is essentially only used for packages that aren't in the official repos. It's not a good comparison, as nixpkgs is the official one.