The decision was not clear-cut back in 2004, and even now, I'm sure that the GTK+ developers would contest this. You also need to consider that back then, people were adapting GTK+ for use on embedded devices, including touch sensitive and more specialised device-specific input. Even now, these things are still possible.
The problem with all UI libraries is that they tend to become their own enclosed universe, which you have to buy into completely. GTK+ suffers from this with GObject and the whole consequences of that. Qt suffers from its complete aversion to the STL--I might want to use it without all its non-standard types. And others like FLTK have a backward and non-extensible widget model by putting all the configurable properties in the base class. I've yet to find one which is just usable without all the extra junk; the GTK+ widget packing model is quite elegant, so it's a shame about the rest.