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human
[Avatar & Legend of Korra] Why did it take 10,000 years to develop early 20th century level technology?

How did it take 10,000 years for society to develop the technology like battle ships and electric lights?  It seems like they were living in decent pre-industrial societies  from when humanity was living on the backs of  Lionturtles to just after the Fire Nation attacked.  Then  in about 50 years people went from 1600 type agrarian societies to early 20th century. 

What was stopping progress for the last 10 millennia?
gpt
I mean you could say the same about a lot of human history on our world. (We've been farming for about 10,000 years, with recorded history going back around 5,000 years). Technology doesn't follow a linear path but is effected by numerous factors of a society. If it did, then China would have industrialized centuries before backwater England, but we know that that wasn't the case. 

As far as the rapid development, I'd credit it to the massive population loss that occurred after the Fire Nations war. Such a loss required investment in labor saving tech and freed up land for more efficient use, similar to the effect the Plague and later the enclosure movements had in Europe.