1. Microsoft has been extremely aggressive in regards to updating people to 10 and many people have walked away and came back to their machine running/installing 10.
2. The 10 upgrade isn't always 100% compatible. I've seen laptops wireless cards get crippled because their software-based wireless switch software quit working and wasn't compatible with 10 or the drivers provided by 10.
3. Windows Updates (including drivers) are forced under 10.
4. The privacy-minded don't care for all the telemetry in 10 which was also back-ported to 7.