Because the nature of economy changed. In the old times, any able bodied person could do manual labour. Now manual labour isn't needed that much (industrialisation, etc...).

Another reason is longer life expectancy and subsidized health care. In the old days, a person inclined to irresponsible reproduction will not live that much longer than a person without. Now there are many countries with an average fertility rate well over 6.

Another reason is that it reproductive responsibility has been shown to work. Compare China to every 3rd world country (e.g. 50 African countries and India). Their one child policy not only stopped untold misery, but created the bedrock for future prosperity.