When Oculus upgraded its system software to Rift Core 2.0, currently in beta, it added a redesigned Home experience, multitasking for apps and windows in Dash and mirrored Oculus Desktop monitors. These are heavy-resource features, according to Oculus, and need Windows 10 to support them. That's why the company is recommending Windows 10 across the Oculus platform (Rift, Touch and core software). You can still use Windows 7 and 8.1, but you won't get the new features as they appear.