Mogrief was a non-starter due to license. Lesstif was a pile of fail forever. Athena widgets? Are you trolling me? See Xfig for an example of how bad those are.
Qt and Gtk were a breath of fresh air. And Qt got everybody's dander up due to 1) license and 2) C++ requirement.
So, Gtk basically stepped into a vacuum.
There were existing solutions, e.g. Motif or Athena widgets if you wanted a GUI library, or FVWM if you wanted a desktop environment. Of course, GTK/GNOME was a significant improvement.