I guess initially Gtk was preferred because of the licensing confusion around Qt while Gtk was clearly GNU open-source. Maybe Firefox for Linux would be based on Qt if it were started today; it would probably help for mobile. I doubt anybody would really be interested in moving an old code base like LibreOffice or Firefox over to Qt, so since they started out with Gtk, they will most likely stay with Gtk.

The last years – since Gnome 2.0 I guess, but definitely Gnome 3.0 – Gtk and Gnome have had an enterprise cleaning broom going through it, which has made them feel more restricted but also more uniform and less as if a bunch of hobbyists have each added their own pet project and preferences to a common code base.

I personally think Qt is great, and with a stronger corporate backer or design team KDE could be polished a bit better without taking away what makes it KDE. Gnome probably has a lot more corporate backing (for desktop) and paid developers than KDE, and I guess consistency and polishing of rough edges are helped by corporate backing and paid development work.

Qt is doing well on mobile though (Sailfish, Ubuntu mobile). It'll be interesting to see how this will influence the desktop side of things.

(For what it's worth – I have both Gnome and KDE installed, and use KDE most of the time.)