Why is it called Windows 10 not Windows 9?

Because they already release windows9 by name windows8.1

I'm betting they expect, at this point, to not increase that number for a long time, and didn't like/couldn't stand being stuck at Windows9 and competing against OS X. Similar to the whole PS3 vs what would have been "Xbox2". They probably really, seriously considered pulling a stupid stunt like Xbox one/Xbox360, but decided that if they did change their mind and keep going with windows numbering that they would screw themselves and make it way harder later. Or maybe they were100% ready to call it "Windows One" and somebody powerful was able to push it out and Windows10 was a compromise.

Windows3/3.1

Windows95/98/ME = Windows4

Windows XP = Windows5Windows

Vista = Windows6

Windows7

Windows8

Windows10?

Windows2000 and Windows NT would not be included as they were focused on the commercial server market, not the home user.

We never have to expect versions to be serial numbers for Windows. Windows didn't follow any serial numbers since beginning. 

First of all, it’s important to note that between Windows3 and Windows7, versions of Windows were designated by a name rather than a number:95,98, NT, Me,2000, Vista, and so on. When Microsoft announced Windows7, there was actually a similar amount of disbelief; after a series of named versions of Windows, it seemed odd to jump back to numbers.

Windows8.1: Actually version6.3, build9600.

There’s also the fact that the name of each Windows release doesn’t actually match thereal version number; for example, Windows8.1 is actually version6.3 of Windows. Windows10 is version6.4. The last time the release name actually matched the version number was Windows NT4.0, which was released back in1996. Windows2000, which was called NT5.0 during development, was actually version 5.0. Windows XP was version5.1. Windows Vista was6.0, Windows7 was6.1, Windows8 was6.2, and Windows8.1 is version6.3. (WinRT, which powers Metro, is a new and separate beast, but it still sits on top of the core Windows kernel.)

Technically, modern versions of Windows are still based on the Vista kernel and code base — including Windows10, which is actually Windows6.4. There will be some confusion if (or when) we eventually reach internal version7.0, but we’ll cross that bridge when we get there.

Alternative theories for skipping Windows9

First, an ExtremeTech reader called Benny sent an email to say that the number9 is considered unlucky in Japan. Microsoft has a big enough presence in Japan that it may have skipped Windows9 to avoid any weirdness or ill will. Benny says that Trend Micro — a Japanese company — did the same thing a few years ago when it skipped version9 of its antivirus software.

Basm Allah Alrahman Alrahim

I think No reason for make that because if you look to windows history you will see that there is a lot of numbers fore some versions look to this wiki list for windows carefully and you will see a lot of strange thing