Hello

In Qt 4 you may make your own theme or style, that is not problem. GTK wasn't elected because it's not so easy compared to Qt 4, not documented well, and much harder to support (e.g. I hasn't success to compile it on Windows)

GTK+ would be an exellent choice, it have its advantages and it is roundly equal to Qt with features. We love GTK+ also, and I personally have Gnome Desktop at home with many GTK+ applications (at least untill KDE4 stable version finally released or gnome migrated to mono ). So, the short answer is "It just happened".