The anecdotal reason (from reddit[1]) was that it would have broken to much third party code doing checks for Windows 95/98.

[1] https://www.reddit.com/r/technology/comments/2hwlrk/new_wind...

Then why not Windows 11? These guys are just not ambitious enough.

I imagine it did play some role but as a child comment notes, the regex checks for Windows 9\d* in order to capture both 95 and 98 in a few languages that exposed versions like that probably played a much larger role. Accommodations that were made for those incredibly old systems could very easily have caused some big problems if they matched on Windows 9.

They probably also thought ahead a bit on how it would wind up getting abbreviated, used, or misused by the public. I recall when the XBox One came out and everybody called it the XBone, several folks on the XBox One team were upset about it.

If there's a Windows 9 it might eventually turn into Roman numerals, so then you've got Windows IX, which could turn into Winix or Wix, or in a particularly bad case "Windows Icks".