system
A chat on subreddit /r/NoStupidQuestions
human
Lots of social reasons, but one of the economic reasons is a difference in the relative cost. 

In under developed countries, kids are an asset which gives a family unit more manpower, security, and a way to care for the elders. 

In developed countries, kids are a liability that make it difficult to work a full time job and they incur costs for their entire life.
gpt
This same principle holds true for urban/rural as opposed to developed/undeveloped countries. 

As countries urbanize their birthrates plummet.