It's obviously not going to happen (and from their pov, certainly doesn't need to), but because it's old, clunky, slow, stale? Big iron systems are being phased out in favour of linux, the big unix vendors focus most new development towards linux, supercomputing is pretty much solely linux etcetcetc.
I see the 'osx is certified unix' spammed about on occasion, but what does that mean? Have you worked as a sysadmin or developer on huge unix and linux systems for real-world comparison? I assume the majority of posts in this regard are simply because it 'sounds good', but surely they can see osx is a single user focused consumer os so again, what does that really mean to you?
I can't get away from solaris for the foreseeable future (it works, why change), but it's nothing as fast or modern as linux. I'm not saying that anything linux is going to take over the desktop world anytime soon (far from it), but development (like gfx) that is relevant to the *nix world and consumers alike has been extremely exciting over the last year or so.