Linux, hard to set up and you can't play many games on it, but you can change out the mobo and processor. Mac, easy to set up right out of the box, no expandibility in the way of new mobos and processors. Windows PC, ready to go out of the box like Mac, but unlike mac, you can change the mobo and processor on a whim (and no command line crap to memorize like Linux. Sure you have some gui programs like windows, but you have to download and install them). Users want ease of use and lower price, so for now, most people buy Windows PCs for two reasons, low cost and gaming is a blast. Macs are a close second in the ease of use, but until the prices come down, and you can switch mobos and processors on a whim, only people that have a lot of money will buy one. Linux PCs are for the educated geeks that are willing to learn a new programming language, command line, and are willing to learn how to recode games to play on this realtively new OS. If I'm wrong, you are free to correct or point out the differences ASAP.

Here's the breakdown in simplest terms:

Windows: Anything and Everything
Mac: Productivity, Everyday Life and Non-Gaming
Linux: Web surfing and after much work you might be able to do some productivity and normal stuff everyone else can do out of the box on Windows and Macs. A lot of work.

Fell outta my chair when I saw Spongebob Mac User

To me it's like Volvo advertising they make the safest cars. You can't help to think that they had to make their cars safer because they had the worst drivers. When you basically try to take a Fisher Price mentality to your computers it certainly goes to say you have the stupidest users so you had to make the thing simpler. Like Del Taco advertising they're cheaper than Taco Bell. Who in their right mind says Hey, I want even less quality food than Taco freaking Bell?!