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.