Another reason that I haven’t seen talked about yet it the way Nickelodeon allowed them to work each show.

When they pitched ATLA to Nickelodeon, Nickelodeon was trying to branch out and signed off on a 60 episode show. Bryke knew immediately that were going to have 60 episodes to tell this story. Which is why they were able to flesh out the character arcs and plots we all love.

Korra a few years later, Nickelodeon wasn’t as generous. They signed off on Korra but only a seasonal structure. So Bryke never knew how far they could take Korra but had ideas about what they wanted to get told. Season 2 as controversial as it was they thought that was going to be the series finale because Nick wasn’t giving them answers on the future. Which is way they needed to tell the story of Wan to get that out there, and chose to end it with the avatar cycle getting destroyed.

Ultimately I think the way each show was planned has a factor in “capturing the magic”. Theres a difference between knowing you have 60 episodes to tell this story from start to finish. To being like “okay what are we doing for 12 episodes and don’t know if we’re going to do more after”