I'm guessing Linux will be a very small % of the overall Rift demographic. It might not make sense for them to spend resources on supporting 1% (NB: random guesstimate) of the user base just yet, although I'm sure that'll happen eventually.
[] If you're still not convinced, let me point out that Oculus supports Android with a separate SDK.
And John Carmack is working on it, and he probably doesn't care as much about OS X. Again, my point is that the constraint here is not the GPU, but Oculus engineering resources.