The simplest explanation doesn't hold much water. I never heard of someone losing data on ZFS but there are whole businesses based on the certainty people will lose data on HFS+. It could be because all the people using ZFS today know very well what they are doing and the same can't be said about the average Mac user, but, still, ZFS seems rock solid.
ZFS the file system, probably. Putting a reliable and fast implementation of ZFS in the OS X codebase may be a different story, as is working out a good UI for migrating users. ZFS is pretty complex, I wouldn't shrug off the challenge of implementing it well.
Second simplest explanation: licensing issues.
Both of these explanations are very very boring, and I haven't seen anything to suggest that any other plausible explanation is very interesting.