Debian/Ubuntu is because for a familiar environment as not everyone knows Nix. [Nix should be optional option to select OS, not enforced. Nix does not follow the FHS.]
More machine types for prototyping apps (and their dependencies) that require high specifications. [because there are apps that are resource intensive to compile.]
Root privileges for control over the environment. [because if someone wants to install X that is not available on any of Nix, they might need root access to install it on the root folder (/).]