Moving /nix to another location would require a lot of package authors that make the assumption of /nix/store being a constant to rewrite their packages, an effort that doesn’t really seem worth it.
The real reason for it are libraries. The binaries that are packaged in /nix/store very often depend on at least glibc and many other libraries. Their path is hardcoded to /nix/store/...
Changing the path would require recompiling, which also means all the caches that were generated would have to be purged and everything would need to be rebuilt.
Though in my limited experience I was able to also use a symlink. I got a warning that some thing could break but all things I was interested in continued to work.