The reasons go much more deeper into the history. China was oppressed already by the British Emperors since the sixteen hundreds. The British sold tons of opioids into China to weaken its population for two hundred years. With the two opium wars they used used weapons to sell their drugs to China mainland and they got Hongkong as a colony. After World War 2 they found another weapon to colonize China and that was communism. So after nearly 300 years of colonialization there was nothing left to feed a huge population in the 1950s which caused the big famine.