It's a driver issue on OS X. Running OS X on the new Mac Pro will also give you the option for 30Hz. When you run Windows on that same hardware it will give you 60Hz so the hardware for it is there, just not the drivers. If you look at the changelog for 10.9.2 you'll see that graphics is one of the changes in that release. We might get proper drivers with 10.9.2 and finally have 60Hz support in OS X as well. Another possibility could be that Apple will save it for 10.10.