The rumors I heard were that SwiftUI was originally meant to launch for watchOS first, to fill in the (sizable) gaps with the UI framework there.
I suspect even though it was always meant to be a cross-platform UI framework, the initial layout system was designed more toward composing and filling a smaller fixed space than for dealing with large resizable windows.
I suspect even though it was always meant to be a cross-platform UI framework, the initial layout system was designed more toward composing and filling a smaller fixed space than for dealing with large resizable windows.