If you go to a computer store and buy a machine, chances are it comes with Windows XP or Vista, and with good reason. If these computers came with linux, well, the world might just end.
When people use a computer, they have a few things in mind, like appearance, usability, and community. When they first load an OS, they want to be welcomed to an easy to use interface. They want an interface that is easy to learn, and simple.
With windows, you get just that. Turn the computer on, set up your account, and the sky is the limit.
The same applies to the mac.