Why doesn't the Visual Studio XAML designer work for Xamarin.Forms XAML files?

Xamarin.Forms doesn't currently support visual designers for XAML files. Because of this, when trying to open a Forms XAML file in either Visual Studio's XAML UI Designer or XAML UI Designer with Encoding, the following error message is thrown:

"The file cannot be opened with the selected editor. Please choose another editor."

This limitation is described in the Overview section of the Xamarin.Forms XAML Basics guide:

"There is not yet a visual designer for generating XAML in Xamarin.Forms applications, so all XAML must be hand-written."

However, the Xamarin.Forms XAML Previewer can be displayed by selecting the View > Other Windows > Xamarin.Forms Previewer menu option.