Because it wasn't developed for those platforms? The code itself will probably mostly be portable but there's tons of libraries that also need to be pulled in. Some programs were developed for Windows, some were developed for macOS, and this was developed for Linux, simple as that.

Windows should be able to run it with WSL2 with some extra setup. If you can get all the other dependencies in place, macOS might also be able to run it, I don't know how compatible the macOS windowing system is with the Linux ones. Same goes for the *BSDs.