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
Lots of mostly rural population are also having lower birth rates as long as women have access to birth control.  

Evolution made a HUGE mistake betting all its chips on making humans want sex way more than we want babies.