[7Z7]

All of the software listed is made by Apple and bundled with the OS. So not that mysterious.

Beyond that, and to your actual point, different developers have different motivations. Maybe their tool fits a specific need on macOS or within their community, or maybe they are starting with what they know, with a view to expand to other OSs later if they get traction. Or they aren't trying to sell the most software, and so don't feel the need to fish in the biggest pond. Or their software is iOS specific, which is potentially more portable to macOS in a way that it might not be to other OSs.

Seems like lots of reasons, and probably not that different to why some software is only available on Linux.