There are a few reasons why games are rarely ported for Mac's. For Linux systems, most people I know use wine, and they can play games with little to no problems (that those on PC's don't have). However, I think that although the question of if there are enough people who would play a game on a MAC, and make it economical for the game companies, the bigger part is probably the fact that APPLE has always kept it's req's for software tightly controlled. That is one reason why in most cases there are few choices for any given type of program. - especially compared to those avail for linux or windows based computers.
I've noticed some games will come out on mac a good year or more after they milk the windows market, but even those are far and few between. The latest craze is more for the systems though - Wii, xbox, etc.