KDE is not worse than MATE.
A full featured desktop environment with software suite compared to a fork of GTK 2.
For strict accuracy, MATE is not a fork of GTK2, but of GNOME 2. MATE did originally use GTK2, though without forking it, and it has since switched to GTK3 (while still keeping the GNOME 2 "look").
I'm not going to say that the project failed entirely because of technical reasons, but at first glance it really looks like they took bad decisions. It's hard to defend a move where you end up with worse software and a worse experience for users, no matter how much money you save.
Whilst KDE is a much bigger project that is actively developed.
Second, you haven't given us any arguments for your "indications of quality" regarding MATE and KDE.
Third, like Gnome, KDE has a huge legacy in FOSS, and is a great project in itself based on a top notch GUI backend. Some of its code even went on and become the basis of the modern web (KHTML -> Webkit -> Blink -> Node -> now also Edge), other tools like KDevelop, Krita, etc are among the best in class in what they do.
What are you, some teenage Linux nerd, with a "favorite" desktop to promote in flame wars?