Well, with OS X, at least, there is no retail Mac that meets the minimum hardware requirements previously announced, except possibly the Mac Pro (though it looks like even that benches lower than the minimum for the Rift). Given that and that Mac graphics driver/OpenGL performance lags significantly behind Windows, there's not really any reason to waste resources on the platform. I don't know what the current state is, but last I was aware, Linux OpenGL performance was also significantly worse than on Windows, so it seems pretty reasonable to me to leave it out as well.
The problem is that most AAA games that are benchmarked on Linux vs Windows are 3rd party ports for a tiny minority among the customers. They put as little resources as possible into the ports, that's just enough to deliver something that is well playable.
Once you can compare an engine/game that has been optimized for OpenGL with equal effort than for D3D, then you can make a fair comparison.
But it's true, OpenGL drivers are often worse. Not just on Linux. Look at what happened when RAGE came out. Presumably that's also only because drivers for D3D are optimized and hacked on much, much more because there are so many AAA games that use it.
That there are already cases where AAA games run at times better on Linux than on Windows, even through a wrapper, is a very, very good sign for the performance to come if only a little more effort is put into the drivers and ports: https://www.youtube.com/watch?v=GZYIa-6UooM
That said, Unity and Unreal are the engines that matter most for VR right now. Both have linux versions. Unity 5.3 was recently released and they have updated their OpenGL backend significantly. I've not seen benchmarks yet...
