That'll be because Windows 10 is the first desktop Windows OS with support for Hyper-V.
If you didn't want to use Windows 10, perhaps you might have some more luck with a Windows Server OS. Does anyone know if the latest version of Docker will work on Windows Server 2012?
Client Hyper-V actually showed up in Windows 8 [1], but only recent versions of Windows 10 have Hyper-V with all the features needed by Docker for Windows.