AppKit was developed for the Mac from the ground up. All effort that went into it was to make the Mac as good as possible. Experience from that went into making UIKit, which was made to be as good as possible for iPhone. Focus on iPhone made the Mac suffer somewhat from a lack of resources, but AppKit was still a rock solid foundation.
Swift UI is primarily made for iPhone. It's secondarily made for iPad. I'm willing to bet that almost all the effort that goes into it is focused on making iPhone and iPad apps better. And it is succeeding there, to some degree (though not without its own issues; especially now with iOS 26). Mac support, however, is clearly an afterthought. Yet it's now the foundation of everything in macOS.
It's not too dissimilar from what it would look like if Apple had decided to rewrite large swathes of the system in GTK when the GTK developers only really care about how well GTK works in a GNOME desktop.