I think Nix is the superior package manager, but you can already usw it with Debian for years now and not many people do it, so the problem seems to lie somewhere else...
1. Too few marketing. I only heard of Nix for the first time last year.
2. When you boot a virtual server, the UI usually offers you Debian, Ubuntu and Centos/RHEL images, sometimes Suse/SLES or Arch. So again, less visibility, and if the hoster does not allow you to bring your own image, it's a pain in the ass to install anything else.
3. (Going off topic for a second: I probably would have switched to Nix(OS) already if it wouldn't entail effectively abandoning the configuration management tool that I maintain.)