KDE isn't the environment of the Steam Deck. The environment of the Steam Deck is Steam running its own Gamescope compositor. The desktop mode runs KDE but that's optional and it's not the main mode because the Steam Deck isn't a desktop, it's a handheld gaming device.
Oh, KDE is the environment of Steam Deck, in case people forgot about it!
So in reality, potentially, KDE is already more widespread than Gnome