In prior times, let's use Vista as an example, we were introduced with the DWM (Desktop Window Manager) which had cool new features like Aero glass. These features are baked into Win32 though, which means you have to P/Invoke to use them from .NET. The .NET team would have to write wrappers around the new Win32 features to make them available in .NET, but because of time constraints, that stuff never made it into .NET API's. WinRT removes this problem by allowing the Win32 API to be directly available through WinRT, so you can just directly access it. This means that in the future if X feature was introduced in the Win32 layer, it could just be exposed through WinRT, and .NET customers wouldn't have to wait for the .NET team to make a wrapper. This is possible because WinRT uses what they call language projections, which allow WinRT to project its API's to specific languages, so you can use the API from your language of choice, and it still looks like you're using your language.
Don't you still have to wrap your Win32 API in WinRT objects before you can take advantage of Projection? And MS doesn't have a monopoly on wrapping Win32 APIs in .Net code. They're just usually the one's expected to do it.