That's because these most of these aren't new kernels. They're just revisions of a previous version, and their numbers are just names, not version numbers.
Windows Vista = Windows 6.1
Windows 7 = Windows 6.2
Windows 8 = Windows 6.3
Windows 10 = Windows 6.4
They are all based on the Vista kernel.
I heard on Reddit that it's because too many 3rd party softwares had "smart" code to read "Windows 9" as "Windows 95" or "Windows 98" to account for syntax errors. By skipping 9, they prevent some software from essentially crashing itself when it tries to run invalid 95/98 code on a new system.