One reason could be that you don't use Windows. Glancing at SubSync's GitHub repository I see it seems to be a Windows project, whereas subliminal runs anywhere Python does.
Its luckily a .net core 2.0 application so it is possible to run on mac and linux. :-) not completely hasslefree though. As you would need to install the required runtime for it.
Is python "natively" supported on those platforms or do you have to install some prerequisites for it to work? If so, it's just as "hard" to run this as Subliminal.
Yes, python is included, though I think macOS uses python 2.x (though people using cli tools will often have the latest). I’m guessing a google search could tell you what is bundled for macOS and various Linux distros.
Is python "natively" supported on those platforms or do you have to install some prerequisites for it to work? If so, it's just as "hard" to run this as Subliminal.