KDE was originally called “Kool Desktop Environment” until they renamed it, first to “K Desktop Environment” then to just “KDE”.
It was the first Linux desktop environment that tried to make all its apps look consistent. Before KDE, Linux UI apps were very inconsistent, each one using whatever UI toolkit it wanted. They likely named all of them with a K to make the naming somewhat consistent too.
Gtk was released two years later, originally only used in GIMP (Gtk originally stood for for “GIMP ToolKit”) and it took a while for other desktop environments to have a consistent look and feel like KDE did.