Say i want to use a UWP file manager on Windows 10. Out of the box the folders i can access are limited, and adding more require that i use the age old Win32 file picker window.
Similarly both Google and Mozilla stopped developing a UWP native UI for their web browsers, as it would only be active if the browser was set as the default one. Something that is not always desirable.
So from a user perspective, UWP feels bolted on rather than a integrated part.