Earlier quoted context omitted.
WPF has a dependency on the CLR which made it a non-starter for the Windows and Office teams. That's the main reason WinRT/UWP XAML was developed in the first place; they wanted a developer platform they could use themselves, and those teams are unlikely to adopt C# anytime soon.
Actually WinRT/UWP are the Longhorn ideas, using the original design of .NET, originally known as COM+ Runtime. Also a reason why .NET is AOT compiled to native code on UWP.
WPF text stack -> DirectWrite
WPF visual layer -> Direct2D, Windows.UI.Composition
WPF UI layer -> WinRT XAML
.net type system/metadata -> WinRT metadata
CAS/partial trust/other .net security stuff -> AppContainer
ClickOnce -> AppX
If you compare what .net was supposed to be around 3.0 with what .net Core is now, they've basically stripped out all of the "OS-like" stuff - UI frameworks, app model/packaging, security - and taken the stance that those are the responsibility of the OS. UWP is just the Windows native implementation of the stuff that was stripped out of .net to be left up to each OS