My understanding [1] is that the software side of OSX graphics architecture presents a set of challenges that are deemed to impede the potential pace of development by the Occulus team. My recent experience with Linux graphics suggests a world of hurt there due to the many moving parts that can be swapped out: e.g. trying to run GIMP while using Xmonad.

[1]: Based on this interview with Andreia Gaita http://softwareengineeringdaily.com/2016/05/05/virtual-reali...