[ashark]

Part of it's probably availability of games on the platform (which has gotten better but is still poor), but part of it's because you can buy both a lower-end Mac (an Air, say, or an entry-level MBP) and build a pretty nice gaming PC for less than the cost of just a gaming-capable mac.

[dasil003]

Also as far as I know you can't bypass OS X's video compositing which adds latency to the rendering, which means that for latency sensitive games you will not be as competitive as a PC.