Plex is a branch that is based off of Kodi, but is nowhere close to having as much power or features. I already have a standalone device running Kodi to this day (currently Ouya, could use any Android device, no need for a server), but if the WiiU had Kodi, I would not need the extra device, could implement the controller to do a lot of functions, and would be able to launch Wiiu games from inside Kodi.
What makes you think no one would be interested in porting Kodi to the WiiU? Just because the wii wasn't doesn't mean the WiiU wouldn't be. The original xbox that XBMC ran on did everything a wii version would have, and was only abandoned because it could not support 1080p, and since the wii could not support 1080p also, it made no since to do a wii port.