"KDE Plasma" can be interpreted as "The KDE organization's Plasma" and probably saves on some article title consistency while avoiding the need to disambiguate the main Plasma article title with (Desktop Environment) or the like. Likely more trouble to try to change than it helps anything as a result.

It's really only calling it "KDE" in isolation that is a bit off. On the GNOME side, such a reference makes sense because the desktop environment is named GNOME and it's run by the GNOME Project/GNOME Foundation. I.e. a bit reversed which word in the order refers to the org's vs DE's name.

Most of the time people will probably figure it out at the end of the day via context either way though.