Because the future of software isn't on Windows. Even Microsoft realizes that with MSSQL moving to Linux (and dying), .Net recommended on Linux and internally, vast majority of workloads that can run on Linux are doing so. Apparently, a ton of Office Teams architecture is Linux.
There is some work to get nix to work with Windows, this is mostly being done by john ericson.
However, the real issue is that nixpkgs (the repository containing all nix expressions) heavily assume unix paradigms.
There could be a "nix-windows repository" in the medium to far future, but it will likely be very divergent from the current nix+nixpkgs story of today.
Well. The future isn't evenly distributed yet :). There is work in flight to make Nix support Windows. Also, and I know this doesn't count, but it works great in WSL today.