OK, so Windows 10 is free for about 65% or more of the existing user base. That is a lot of people.
I think that the relatively low, now almost flat adoption rate for Windows 10 is that people don't like the 2D interface. Plus, it is not as stable as Windows 7 and it has crap stuck on the start menu that you need to fiddle with to get rid of. It also has more "telemetry" built in. None of the so-called improvements are particularly useful (Cortana is a great example). It seems dumbed down so that it will work on mobile devices. I think that is why there is a market for the older version that has more features (Windows 7 Ultimate), which doesn't have all the downsides.