Linux gamers are actually a bit more numerous than Mac gamers, if Quake Live is any indication. And they seem a lot more starved for good games (see average money paid for Humble Indie Bundle). And contrary to what some people say, you don't have to spend a lot of effort supporting multiple varieties of Linux. Linux users are exceptional at helping themselves and if you target only one distribution (like Ubuntu) the pack leaders will translate the process, write instructions or even install patches.
I have been paying for my own PC games approximately since the late 70's.. Back then I would have to search phone books and drive over 60 miles to find a store carrying PC games.. In these stores there would be some games for commodore, a few (very few) mac games and a far larger varitey for windows and dos (while dos lasted LOL).. This is simply the best representation I have to illustrate why I suspect there is less support for the mac/linux os systems..It is not just that a company might not sell enough games to cover the costs (let alone make a little profit) it is also that the shelf space (both virtual and brick and mortar) would be less which then leads to less sales, less profit and so on..
add to the fact that Joe public computer user, has (as a rule) a limited skill at using his operating system let alone one less user friendly (IE requiring more knowledge of whats going on the put the disk in and wait for auto install).. God help me my mother's email program just added a spam filter function and its driving me nuts teaching her to use it.. but i digress, it is largely this reason that keeps Linux/MAC in the realm of the competent PC user and there for in the lesser used market.. this in turn leads to fewer companies spending money on more the one OS when one will clearly net then profits and the other most likely wont..