Huh? Qt, GTK, Cocoa (AppKit and UIKit), bunch of other linux friendly gui frameworks been around for a long time, even Flutter is still around. What is being killed?
What versions of QT and GTK? Running GTK1 or 2 apps is pretty hard. P TUI apps from that era work just fine!
The same reason webui and js is so popular!
gtk1/gtk2/gtk3 code will require you shipping gtk(version) on modern linux, not all distros have the legacy libraries.
Apple deprecated carbon (which was a thing when gtk1 was around). I don't think you have an option for this on their ARM hardware.
QT1->N code has the same problem, the older libraries are not shipped on most modern linux.
I do absolutely understand if you're going to do static compiles, that can work around that problem, but that arguement nullifies everything, since you can run/write/execute anything in a turing complete system, if you complain you're just not dedicated enough.
Apple believes SwiftUI is the future, not AppKit and UIKit/Cocoa.