Most of the private users, companies (apart from the graphic designers) use PC. Most PCs these days have Windows installed.
Why? Most software in the market are only for the Windows operating system. Most of us use Microsoft products such as Microsoft Office used to be only compatible for Windows. Although Microsoft Office can now be executed in a Mac environment, most of us are so used to using PC, it is difficult to switch to a totally different operating system with a different environment.
For commercial software development, most developers use programming languages that are created, compiled and used in a Windows environment. It's not very profitable making MAC software as most of the target users use Windows on a PC.