[jbarrs]

Because Microsoft almost have a monopoly on the OS market. Pretty much the only alternative for users who aren't tech savvy enough to comfortably use Linux is to buy a Mac, which will price out a lot of people. Even if you are able to use Linux (or can afford a Mac), the popularity of Windows means that you will eventually run into a piece of software you need that isn't available for your OS and won't run in Wine, forcing you to use Windows by some other means.