Developers want one streamlined coding or engine in all their games. If they want a MAC game, they have to find someone with UNIX and UNIXy knowledge capable of recreating the game for MAC and also using the UNIXy knowledge for a multiplayer enviroment. It takes a lot but I would like to see all games work for all major OS systems and not be bottle necked to just Windows.
You don't need to convert your mac, just run the new Snow Leopard OS, and set up a dual boot with bootcamp and make Windows your second OS. Then you can use your MAC to play games in the windows boot. Then you have the best of both worlds.