I think the reason most games don't come out on Mac and Linux is because, well... Windows is the most popular. If you ask your average person about linux they'll stare at you for a few seconds then walk away. Or maybe they'll pronounce it wrong and then show you some freakin' picture of Tux with a gun and the windows logo. And they'll call him linux or the linux penguin if you ask what he is.
Also, the majority of Mac users don't buy Macs for games or are even gamers at all. My Mum uses a Mac laptop because she teaches and all the computers at her school are Macs. Because they're idiot proof which is why they're bought mostly.