I wouldn't necessarily call their platform a niche - but rather one that doesn't foster a lot of open-source contributions. I've always felt like development on Apple's platforms, even when wildly successful, is often a close-sourced and proprietary endeavor. It doesn't surprise me much that there isn't a big footprint of their languages on GitHub.
Yeah, I think there's more of a culture of paid (and therefore mostly closed source apps) on Apple platforms. Whereas anything cross platform is _not_ using Apple languages for the bulk of their code base, and Linux focused/exclusive apps are going to be over represented among open source because people who value open source are more likely to use an open source OS
