It was/is a great tech, but has MS used it in any of their own stuff besides Visual Studio and the discontinued Expression suite?
Cocoa/AppKit is used throughout macOS and all of Apple's own apps.
The last time I was in the Microsoft galaxy, WPF had been dogfooded in barely 2 of MS's own apps, Silverlight had been deprecated, and the new "Metro" and WinRT or whatever the new platform for tablet-like apps in Windows 8 and 10 was called, had all but killed WPF. Maybe someone who used to actively develop WPF apps can shed better light on its fate – I'd almost love to be wrong on this.
On the other hand, apart from having a vector-based UI (though I suppose they don't need a vector UI; having solved high-resolution displays better than Windows' scaling does) Apple's development technologies were already everything Microsoft wanted to be, and apparently admitted as such in their own internal emails. [1]
Indeed, to my knowledge pretty much the entire userland of macOS/OS X is in Cocoa, from the Dock to the Finder to the Automator, and even their Big Apps like Final Cut and Logic have been made with Cocoa.
I can only assume you never heard of WPF.