They won't tell you why, let me commit suicide by guessing it
Windows have DirectX 12 which is said to provide performance increase on same old hardware just by being there, most mature OpenGL drivers provided by vendors and Microsoft's good developer support.
I used Apple as my only system for everything before bootcamp&Intel thing happened, I know couple of things about its OpenGL support. It is always behind the curve, Apple has horrible developer support compared to Microsoft. Ask any die-hard Apple gamer who runs same game on both OS X and Windows and you will be surprised about the performance difference.
This "protype sold to consumers" type thing requires perfect performance by host operating system. Especially Graphics performance. Think it like a game.
I am not saying anything about Linux but I bet Steam will change things in future, I thought I better target Apple since they make money selling software&hardware.