Marvel licensed shows and imported animes are not Nickelodeon productions and are universally relegated to Nicktoons Network. Avatar and Korra are Nickelodeon Studio's own productions, and air new episodes on the main Nickelodeon network. Although I suppose it is worth noting that before Avatar aired, Nickelodeon never really dabbled in importing anime and licensing superhero shows. Maybe Avatar's success spurred them into trying those shows out.
Originally Posted by tealeaf
And while they do have lighthearted moments, the last episode of Korra really was that dark.
I AM ASSUMING CONTROL OF THIS FORM.