It's because Nix wants to install into /nix. Once upon a time doing "sudo mkdir /nix" wasn't a problem, but recent macOS releases have made that very hard.
Nix could switch to an alternate location on macOS (e.g. /opt/nix) but that has a lot of downsides for interoperability with other systems.
Nix could switch to an alternate location on macOS (e.g. /opt/nix) but that has a lot of downsides for interoperability with other systems.