Is it simply because there is AskDifferent and MacOS questions should be pushed that way? And if so, perhaps it could/should be referenced from the FAQ
It might not be a bad idea to reference AskDifferent in the FAQ but I don't believe that's the primary reason. I've read elsewhere on meta.stackexchange.com multiple times that sites generally should not define their scope based on the scope of others.
As I understand it, it's much easier to explain in terms of communities or readership. That is the scope is based on who U&L is for than what it's for.
Unix and linux distributions have a very large set of overlapping software, and consequently their users have a very large overlap in skills and knowledge.
An experienced user of Ubuntu is likely to be able to jump onto a Centos server, or a FreeBSD laptop, or even an AIX server and get things done. They are close enough and many / most differences are a short google search away. So asking questions around any of those OS are likely to fall cleanly within the knowledge and interest of the readership here.
But that's largely based on those distributions comprising of a similar set of open source tools.
So this leads to two major exceptions:
- Android (Linux)
- MacOS (BSD)
Both of these lean heavily on the same set of open source tools, not just the respective kernels. So you may well get a good answer asking about a ZSH on MacOS question or an OpenOffice question here because the U&L community have experience in those tools.
However both Android and MacOS have their own individual platform built on-top of the open source tools. Amongst other things the platform includes the GUI and heavily customised configuration. So the U&L community are much less likely to have specialist knowledge of those aspects.