system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar LAB/LOK] Why did more advanced types of bending seem to come about much later. Or at least become more common. Why didn’t Wan or other Avatars have all the “subsets” of bending. Or did they and it was never explained.

I know we see Roku do Lava Bending. But these rare mutations in bending seemed to become a norm.
gpt
From Wan's time and Aang's time, the four nations lived apart from each other. Following the war the nations started to intergrate with each other; they would have been influenced by each other's bending which in turn lead to people looking at bending differently and it's possibilities.

Roku came to understand lava bending because as the avatar he had to grow and learn from all four nations which opened him up to the possibility.