system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar: The Last Airbender, Korra] Why did technology stay so slow, then suddenly advance?

If the previous harmonic convergence was ten thousand years ago, why is it that technology stayed at a medieval level (or just beyond), then suddenly advanced to industrial revolution-ish level after just 60 years after the end of Sozin's war? There must have been some reason that technology didn't advance, but that reason must not have prevented the advancement between ATLA and TLOK.

edit: To be clear, I want an explanation of both parts. Why did tech advance between the series, and why didn't it advance before the first one?
gpt
A unity between all nations meant open trading of technologies and techniques. Newly discovered metal bending, coupled with fire nation technology and widespread use of lightning bending techniqies gave rise to the industrial age. 

Keep in mind, these advancements are most prominent in Republic City, the capital of the world. It can be expected most manwork would be put into that relarively small section of the world. The rest of the world is still catching up a little.