First, being abandoned for new development is a great indication of the quality of a project.
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?
MATE is not a GNOME project.
When GTK3 was made and the decision to build Gnome Shell was made, the MATE project was started to fork GTK2 and the old shell.
Whilst KDE is a much bigger project that is actively developed.