There isnt a specific answer to this question. Microsoft has never been coming with a clear answer for this query. There are a few theories being put forth in the technology circles about the reasons for the same.
One of them is interestingly enough that number 9 is considered unlucky in Japan. Microsoft apparently does not want to go wrong with its prospects in Japan.
Also Windows 10 is not just an update to Windows 8. It has been developed as an all encompassing operating system including WindowsPhone, Windows RT and Windows Embedded. As such Microsoft might have thought of naming it with some strong figure and the number 10 gives an impression of fullness.
Howeve, at the end of the day, all these are just beliefs and assumptions. There doesn't seem to be a concrete reason for for the lack of Windows 9.
Let me provide an Important information here, though off topic to some extent. Windows names are never their actual version numbers. Windows 10 is not actually Version 10, but it is 6.4! Likewise Windows Vista was version 6.0, while 7 was 6.1. So, what's in a name, let's enjoy the new features as long as they stay to be better.
Live....and Let Live!