Someone claiming to be a Microsoft Developer has suggested an explanation for the skip from Windows 8 to Windows 10.
Microsoft dev here, the internal rumours are that early testing revealed just how many third party products that had code of the form if(version.StartsWith("Windows 9")) { /* 95 and 98 */ } else { and that this was the pragmatic solution to avoid that.
http://www.osnews.com/story/27969/Why_did_Microsoft_skip_Windows_9_