I think a lot of people come to Nix and NixOS from Linux and similar environments, where having "repositories" or "registries" is fairly common as a way for distributing indexes of software in their distributions. So it's quite naturally moving in that way.

But for someone coming from OSX/macOS or Windows where there basically is just one index (provided by the companies maintaining the OSes) and you can't really add/remove others, it's a completely new concept, makes sense there is at least a bit of friction as those people wrap their head around it.