Windows X is more catchy, than Windows 10. :/
Microsoft's naming conventions are bizarre. Even the Windows 7 is not exactly the 7th version of Windows OS, it's the build of Windows 6.1
Check in Windows products like Calculator, Notepad, etc., Select HELP and then ABOUT, and see the version there.
Windows X reminds me of X-Windows on *nix machines. That's why they probably didn't choose it.