Well, in this case, it probably has something do with the word "Nickelodeon"( :P )and the fact that most long-running adult-aimed animation in Western countries is either based on long-established iconic characters and/or much more gimdark than what they're aiming for(or a variable mix of gross-out humor and topical social satire).
Besides, the mastering of the elements has always been something of a backdrop for coming-of-age stories. You could make a story about a grown-up avatar who's already mastered all of the elements, but you would likely have to go more for character-driven tension or something more philosophically or politically driven since actualized avatars are sort of imposing at physical conflicts.
*shrug*
Maybe a third series could involve an adult Earthbender avatar dealing with how to maintain peace. Or being reminded of the dangers of becoming too set in one's ways. That might fit thematically.