The number of children people have is heavily influenced by economic factors: poor or insecure people have a lot more kids than wealthy or secure people. As soon as the overall prosperity (along with a few other factors like urbanization and healthcare) reached a certain level where the fertility rate would subside, the policy was rolled back.

After a certain point, there are aspects of liability with a greater population. More moiths to feed, more clamor for goods, but beyond a certain threshold it doesn't give you any more economic or cultural diversification. On the personal level, each person loses some of their their distinction and individuality when there are more hundreds of millions of people out there.