While I agree about the respective qualities of the toolkits, if I remember correctly Gtk wasn't initially a replacement for Qt, but for Motif. The first version of Gimp was using that, and there was quite a bit of animosity against C++ back in those days.

They still could've tried to contribute to LessTif or at least created a new Xt-based toolkit instad of creating yet another overly verbose C OO system...