Last year Iran was hit with restrictions on actually selling its oil. This cut oil income in half and Iran is scrambling to invent novel ways around these new problems. Slowly, Iran is coming up with new tricks to evade these barriers to its oil sales. North Korea has no such similar exports to sanction. North Korean exports are largely illegal items, like drugs, counterfeit American currency, and weapons (especially ballistic missile and nuclear weapons technology).
Both countries pay well for the services of their smugglers, who need that kind of incentive because a growing number are getting caught and sent to prison.