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.