It was to encourage population growth. Sex selective abortion had nothing to do with the decision, as at the time that abortion was initially forbidden in China in the early 1950s, there was no way to determine sex before birth.

The first way to determine sex in utero was amniocentesis, which was invented in 1956 in Europe, and wasn't commonly used in China until a long time later--that level of advanced medical technology was beyond the means of the vast majority of Chinese people at that time.