If I'm writing for a proprietary OS, that's probably Windows or OS X. Both of which give me a more stable target to aim at when it comes to what dependencies I can expect.

If I'm writing for an open source OS, that's probably some flavor of Linux. But which flavor? And which package manager does it use? And do they support all the dependencies I need, in the versions I need? Without pulling non-standard package repositories into the mix?

When open source software has difficulty working on proprietary OSes, I think it's usually not because of the package manager. More often it's because of something like a glibc dependency. At which point the domain of support isn't really "open source OSes", it's usually something more like "glibc-based linux distributions."