[Snargorf]

There are reasons.

China has a long history of advanced (for the time) civilization and a high-IQ population (~105). They were backward for just a few centuries because of a series of bad ideologies - isolationism, followed by communism. For most of history China was the highest-tech civilization on Earth. All they had to do to achieve demographic transition was get back to their historical norm.

Africa has no history of advanced civilization and a low-IQ population (~75). There's reason to think they'll sustain higher birthrates than everyone else, indefinitely. To achieve demographic transition they'll have to do something they've never done before.

[int_19h]

Given that IQ scores correlate pretty well with things like child nutrition, I'd wager that if you could actually go back and measure them for China back when they were "backward", you'd probably get similar results.

Not to mention that IQ is not an absolute metric - it's the median score of the entire population, and we're getting smarter as a whole, so 100 points today is "worth" more 100 years ago. Indeed, if you use the modern metrics, and apply them to the population of US - a well-developed industrialized country at the forefront of economic, scientific and technological advance - back in 1920, the average IQ would have been 80. So, if, as you claim, ~75 is such a low IQ that it would preclude these developments, then US should not exist as it is today.