Windows 8 was just a horrible mistake. It was a good NT operating system "under the hood", but the new smartphone-inspired user interface ("Metro") was beyond the pale on a desktop. It was denounced and rejected almost universally.
Windows 8.1 addressed a few of the most egregious flaws in Win8, but didn't begin to fix the overall problem.
Windows 9 never happened. They went directly from 8.1 to 10.
There is precedent for this. When Windows NT first came out, it was version 3.1 to match the then-current MSDOS-based Windows 3.1. There was never a Windows NT 1 or Windows NT 2.
MSDOS-based Windows:
Although technically correct, Windows NT 3.1 was based on OS/2, its predecessor. So although there is no direct correlation about a numbering scheme of NT version 3 following OS/2 versions, indirectly one might say there is (very loosely). I'm a former NT/Windows Server admin and supported OS/2, NT, and Windows (in addition to flavors of DOS back to version 1) and still have various copies of the software including a shrink-wrapped new box of OS/2 version 2.0 (early 1990s) that I never opened.
My understanding is that they skipped "9" because of existing code in the OS, utilities and some apps that checked for the Windows version by only looking at the first numeric digit. So Windows 9 would have been confused with Windows 95 and 98 in some cases.