When you are getting an OS, you basically have only 2 choices: Windows or some form of Unix.
With Unix, there are two main variations: Mac OS X and the most popular form of Linux: Ubuntu. Of course, there are many other variations, but they are less popular.
Microsoft owns more than 80% of the business, and with Apple taking up what little else, Linux isn't a competitive player in the market. All of these operating systems are borrowing ideas from each other and trying to innovate faster and improve their products more efficiently than their competitors. That's when it's just Microsoft vs. Apple. Linux is mostly freeware, and it looks like it's going to stay that way.