The problem is SwiftUI. It's very new, still barely usable on the Mac, but they are adding lots of new features every macOS release.
If you want to support older versions of macOS you can't use the nice stuff they just released. Eg. pointerStyle() is a brand new macOS 15 API that is very useful.
They are working on it, and making it better every year. I've started using it for small projects and it's pretty neat how fast you can work with it -- but not everything can be done yet.
Since they are still adding pretty basic stuff every year, it really hurts if you target older versions. AppKit is so mature that for most people it doesn't matter if you can't use new features introduced in the last 3 years. For SwiftUI it still makes a big difference.