Gtk's popularity comes from a couple of things that happened about 20 years ago...

- C++ dynamic linking used to be slower on Linux than C dynamic linking

- A lot of Linux die-hards are simply C++ haters

- Qt's licensing used to be unfavorable to purists, which lead directly to the Gnome-vs-KDE schism.

(Though I've always had the feeling that the license fight may have just been a palatable cover for a C-vs-C++ fight.)