Neither of the existing shows featured adult Avatars trained and ready for world-ending threats - Aang and Korra were both young and still mastering the elements when their villains forced them into conflict.

Source: https://screenrant.com/avatar-last-airbender-new-show-aang-adult-story/

Set in the same universe, years after the events of the original Avatar, The Legend of Korra is a far more adult version of the show, dealing with surprisingly mature themes.

Source: https://www.cnet.com/culture/entertainment/the-best-fantasy-shows-to-watch-on-netflix-in-2023/

With the goal of dismantling all world governments and ending the Avatar Cycle, the Red Lotus would prove to be one of the greatest threats to peace during the era of Avatar Korra.

Source: https://gamerant.com/avatar-the-last-airbender-white-lotus-explained/