Why did the concern of population growth dominate prior UN ICPD conferences?

After a long period of history with unremarkable changes in population growth, the world population doubled for the first time from 1 billion in 1800 to 6 billion by around 1930.

By 1960, the population reached the 3 billion mark, and
Between 1960 and 2000, the population doubled again, reaching 6 billion.

This fantastic pace of growth is estimated to continue into the foreseeable future.