It's because of the culture, economic situation and standard of living in most white majority countries. Individual happiness is very important in the industrialised world. A child could get in the way of going out and getting very drunk, high, clubbing, pubbing, cinema, theatre, restaurants etc. Life is expensive so bringing up a child will cost a fortune. Also people are changing, a lot of people just don't want children and they are no longer expected to. Secular and atheist belief is high and both don't require children if anything it discourages it due to overpopulation of the planet.
Contrast that with the developing non white world. Most are still religious which promotes big families and is anti family planning. Many still have big families due to high infant mortality rates and the cost and standard of of living is low. They consume a lot less resources per head and don't have many of the luxuries taken for granted in the industrialised world so have no idea what they are missing out on. The concept of individual happiness is nowhere near as important as doing what the culture of that society expects of you (i.e get married and have children).
