Why does North Korea have food shortages?

North Korea is not so much a communist country, but a despotic monarchical dictatorship, which strange as it sounds, it is nothing unusual to Koreans, considering Korea's dynastic history. Other communist countries where able to successful implement collectivization and centralized distribution, which did help rural people to a certain degree. This has not taken place in the DPRK, as most of its produce is sold abroad for funds, primarily to China, while those in the lower rungs of society, the peasants, starve. This lack of food also affects urban population, and is clearly visible when one compares the lesser height and weight of adult North Koreans to that of the much better fed South Koreans.