The impetus to create the GNOME desktop environment was KDE's reliance on Qt from Trolltech, as well as a preference for C over C++. At the time Qt was available in source form, but modifications could not be distributed. GNOME adopted GTK (or GTK+ as it was then) from the GIMP project, and this eventually encouraged Trolltech to re-license Qt to allow distribution of modifications. And now twenty years later Qt goes closed source :-(
 It's far easier to create bindings for other languages if the library is in C rather than C++.