Some users are aware that the free upgrade from Windows 7 SP1 and Windows 8.1 for private users (in the home and professional versions) to Windows 10 still works. Now there is an unofficial explanation.
I think the answer is simple: Because nobody would pay for an upgrade to Windows 10. This sentence from the article is the most telling one:
WDG didn’t care pretty much at all because Terry Meyerson at the time cared more about his upgrade stats than license revenue as Windows isn’t Microsoft’s cash cow anymore.
Microsoft’s management, and in particular Nadella, is deeply worried that it becomes obvious that they have no idea how they could improve Windows in a way that people would actually pay for it. This is why they have to force their bugware on customer PCs just so that the “upgrade” numbers look good.