Macs have sucked, do suck, and likely will suck for all time to come as far as gaming is concerned. They ship with shitty graphics cards, have bad cooling, and there's simply not much in the way of interest from Apple in supporting gaming.
I could easily see "casual" PC gamers (people who play games casually, not who play casual games) buying a Mac for their general purpose PC and playing games on the side, iff the Mac offered decent gaming performance (they don't) and were priced around the same as a decent gaming PC (they aren't).
It's a chicken-and-egg problem, though. Studios don't want to invest in making (or even porting) games for a non-gaming platform, gamers don't want to buy a computer they can't play games on, and Apple doesn't want to invest in pleasing a consumer segment that doesn't exist.