Why Windows 10 released instead of Windows 9 ?

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.

There are multiple theories with no fixed answer to this.

One theory suggests that Windows 10 is the last desktop build for the Microsoft. And from here on-wards Windows 10 will be updating the OS incrementally in the background. This makes Windows 10 cycle just like Apple's OSX. And as it saves them from spending millions of dollars in advertising each new version of OS.

Another theory suggests that Microsoft released the Windows 9 in the community release of the Windows 10 OS. And the successful builds were moved as final version for the 10.

Most of the tech journalists are reporting different theories. So I guess it's upto us to decide which theory is more close and official depending next few months moves of the Microsoft inc.

Personally I found theory of making build cycle similar to Apple OSX sounds more close to being official reason for why Microsoft decided to choose Windows 10 as it's release name.

There was Windows 97, Windows 98, Windows 2000, but Windows 99 was not so popular. May be that's the reason, they skipped towards a complete number indicating more advancement and completeness in its features.