Why hasn't XNU kernel been extensively used?
On one hand, Linux had a considerable head start on XNU, having been first released in 1991. While I can't seem to find information on a release date for XNU, I'm quite sure it happened after Apple acquired NeXT, in 1997.

Also, the kernel design used in XNU may not be easy for some developers to get their head around. Linux is a monolithic kernel, while XNU is a hybrid (a cross between monolithic and micro kernel models) It's possible that developers are more comfortable developing for the former model than the latter. Ken Thompson, one of the fathers of UNIX, also argued this point.

More information can be found on Wikipedia (starting at Kernel)