Why did they skip Windows 9?

According to Reddit user cranbourne, Microsoft decided to forgo Windows 9 on account of legacy code from third-party apps running checks for Windows 9x. and that this was the pragmatic solution to avoid that. It’s almost like Microsoft’s version of Y2K.

What is Windows 10 called?

Microsoft is officially unveiling the name for the next release of Windows today: Windows 10. While the software maker had referred to Windows 10 as codename Windows Threshold internally, today’s official naming puts any rumors of Windows, Windows TH, Windows X, Windows One, and even Windows 9 to bed.