This a good point, and I agree that C++ made sense at the time as it is much richer for creating a GUI. I've never tried to use GTK C GObject interface, but I've seen some snippets and it looked nasty to me. However, in hindsight, it at least makes it fairly easy to create bindings for and I suspect many (most?) don't use the GTK C API but some other language bindings, so it turned out to make a lot of sense I think.