[kmeisthax]

Windows gets to completely rearchitecture their compositor because they only provide one stable ABI to get pixels on the screen: link to USER32.DLL, create the necessary objects to represent a window of your application's class, then create and pump a message queue for it. It's ancient, but it works, and more specifically will never change. Even the higher level toolkits Windows ships ultimately are creating USER windows, and USER has been the only UI ABI since version 1.

macOS is the same way, except Carbon (a light modification to the procedural Toolbox API) and Cocoa (the Mac's first OOP toolkit) were "toll-free bridged" to each other rather than, say, writing Cocoa in terms of Carbon.

In contrast, X11 is a protocol anyone can implement and speak. There is no blessed library that you must use. No, Xlib doesn't count. Servers have to take their clients as they come. And Wayland, while very much deliberately stripped down from X, still retains this property of "the demarc point is a protocol" while every proprietary OS (and Android) went with "the demarc point is a library".