Microsoft decided to skip Windows 9 because, after doing a lot of research, they found that a lot of commonly used legacy software had implemented compatibility hacks which involved checking for “Windows 9” to detect when the software was running under either Windows 95 or Windows 98.

Instead of breaking a lot of software or requiring a lot of updates (some of which could even be from vendors who were no longer in business) they decided to work around the problem by just skipping straight to 10.

Edit: My mistake, I responded to the wrong comment. But I’m gonna leave it here because I already typed it.