The sanctions go back to the war, the famine after the war were due to having most of they buildings destroyed and 20% of their population killed. They restarted after the USSR fell and they lost their biggest trading partner, same story with Cuba.
What's the alternative story here, that Kim Jong Un eats all the food? That being "aUtHoRiTaRiAn" means he would rather rule over a starving populace than buy fuel for the tractors?