Technologically, there's no reason why a Mac can't run games. There's very little if any difference between the hardware on a Mac and a PC these days, especially with Mac using intel inside XD
I think it comes down to user base. Take that whole pie of potential game running hardware of which even Windows PCs are dwarfed by consoles. Look at the Mac slice of the pie and it's very small now look at the Mac user base that would even want to play games and it's smaller still. Crumbs even.
Wakka wakkaI'll explain Blizzard by using these two analogies. It's like saying there are people in the capital city of a large foreign country that speak your language, but as soon as you set foot outside the capital nobody has a *dancing* clue what you are saying. Or a musician being able to claim they are 'big in Japan'.
So it's possible to be 'big in Japan', no-one else gives a *dance*.
Oriental dudes discussing the great 'Mac as a games machine' debateEDIT: just had to add this link
http://www.penny-arcade.com/comic/2006/3/3/