When Shri Krishna established a huge city Dwarka & became Dwarkadheesh (Emperor of Dwarka), people under his protection became all powerful & ruled the Earth. Then Shri Krishna made sure (by helping Pandavas throughout) that morally corrupt & debauched Kauravas of Hastinapur (near today's Delhi) got their just deserts for their evil deeds like trying to disrobe queen Drupadi in front of everybody in the court. But after kauravas were gone & under the twin rulers of Shri Krishna in Dwarka & Yudhisthir in Indraprasth (very near today's Delhi) the people in Shri Krishna's kingdom were so powerful, that Lord realized that if I am to go leaving any of them behind, they may end up as bigger threats to dharma & dharmic ways of life than even Kauravas. So I'll make sure that all of them are gone before my departure.