Because it was over populated and if later on in life they did not introduce the one child policy then they would not have enough food to provide for the country. This will cause people to starve of hunger.
Why Did China Introduce The One Child Policy?
The fertility rate in China before the policy was introduced was, at one point, as high as over 5 births per mother. Seeing the example set by other LEDC's (less economically developed countries) who decided to ignore the problem with bad consequences, China decided to act, introducing this policy to combat overcrowding.
The less the children the more money they save