People have wondered: "What happened to Windows 9?"
Well, there are a lot of lazy programmers in the world. When they write code to determine which operating system is in use they often search for "windows 9" or "windows 9x" to see if Windows 95 or 98 is in use. Now if Windows 9 were in use the program would think it was windows 96 or 98, which are not compatible with Windows 9, so unexpected things would happen. So Microsoft was forced to skip over 9 and go straight to 10.