I will refer applications in Windows Store for either Windows 8/8.1 or Windows 10 as UWP applications as Wikipedia does, if I do not mistake it. Windows RT(not WinRT though) is not discussed to avoid confusion.
I wonder what API's no matter public or undocumented that Windows 8/8.1/10 have implemented that Windows Store and UWP applications rely on, so that they cannot be port to Windows 7, which is NT 6 as Windows 8/8.1/10 do.
If nothing much are there, but only due to M$'s market strategy to promote new technology'd Metro, there might have been some 3rd-party runtime/framework for Windows 7 on which Windows Store will work and UWP applications can run, which will surely spend no more effort and cause no more legal or compatibility trouble than Mono or Wine.
Edit: As How does Windows 8 Runtime (WinRT / Windows Store apps / Windows 10 Universal App) compare to Silverlight and WPF? says, it's a layer quite similar to .NET that lies between COM and application, if I didn't misread.