It is not much of a question of what's wrong with GTK, Qt, etc.

Flutter is cross-platform. Canonical is doing the work for Linux. Toyota is doing the work for embedded devices (to use in their cars). Microsoft is doing the work for their foldable tablets. At the moment, Flutter has the momentum to be the UI for everything. Whether it will pan out, we will be here to see.

Bad choice, but logic is easy to understand. Gtk was killing Ubuntu, and they chose to be killed by Flutter instead.

The bad about what Gtk is now is it has turned from the "Gimp Toolkit" to "GNOME Toolkit" with upstream thinking that's how it's supposed to be, and unwilling to rectify.

Flutter being a mess as it is, comes after it completely, inherently unsuitable for Linux, and desktop use.

Maybe Canonical is planning a mobile reentry.