Probably because importing Mach messaging and VM APIs into the Linux kernel, and recreating IOKit and similar from scratch is a pretty major undertaking; the APSL isn't GPL-compatible, so using the existing XNU code and trying to make it fit into the Linux kernel isn't something that could ever be upstreamed. Aside from the fact that Linux's device model doesn't exactly map to IOKit well.
Then there are invariably plenty of assumptions that userland makes: launchd is probably assumed to have PID 1; all sorts of stuff like that.
Plus of course, there's the question of whether this would be inline with the EULA of Apple's userland…
There's always GNUStep if you want to write Cocoa apps in Objective-C on Linux. Somehow I have a feeling the APIs aren't what's been holding back Linux on the desktop.