Hihi!
Some of the benefits of the one-child policy were that there wouldn't be as much over populating and over-crowded. There was room but not that much for millions of people to be in one area so that's a huge reason why. Another reason would be food. They just didn't have enough food to feed that many people, and if they were to trade for more food and other necessities for all those people and children that would cost the government way too much money to feed and house everyone.
I hope I helped!
-Jailbaitasmr