I've often wondered why BSD is not more popular as a desktop system. Since it seems more "stable" it would surely make an easier target for development?
There must be a reason Apple chose it instead of Linux to be the core of OSX.
OS X is not really a BSD. True, its userland comes from a mish-mash of NetBSD, FreeBSD, classic big-U Unix, and OpenBSD, in roughly that order, but xnu is not a BSD kernel, and while it supports many FreeBSD APIs (e.g., kqueue), that's done by reimplementing those APIs in xnu, frequently via gratuitous copy and paste, but definitely not by using a BSD kernel.
I mention that not because I want to be pedantic, because there's a huge gap between "OS X is using some BSD components" and "BSD would make a great desktop operating system." The one doesn't imply the other.
That said, there is a fairly nice desktop OS, called PC-BSD, you can take a look at, that is FreeBSD with a KDE-based UI. Not my cup of tea, but you can check it out at http://www.pcbsd.org/
Apple couldn't choose Linux as a core of OSX or iOS even if they desperately wanted to, because GPL would expand to entire "collective work" including closed-source userland tools (if they are distributed together with a kernel as a single downloadable and installable package).
If Linux kernel would have been licensed under LGPL, or CDDL, then it would be possible. But GPL is too viral for such thing to happen. It was specifically designed to crash the barriers of closed source code and expand itself to entire derivative or collective works.
> Since it seems more "stable" it would surely make an easier target for development?
That's not really an accurate statement - you can have very stable Linux distros, and you can have very unstable ones - same goes for installations of the BSDs. (Not to mention that 'stable' is in itself an overloaded term).
One may be better for an individual use case, but I don't think it's possible to say that BSD is more stable than Linux (considering that you're not even comparing like quantities there!)
Although there's little to no doubt that Linux was evaluated for Rhapsody (and probably rejected for licensing reasons), another explanation is simply that Free and NetBSD represented a more direct upgrade path from NeXTSTEP's existing 4.3BSD codebase. Since the goal was to bring NS into the Apple ecosystem and explicitly not to create a new OS altogether it would make for the obvious choice.
There must be a reason Apple chose it instead of Linux to be the core of OSX.