Why did the early Russian state of Kiev flourish?

If you are looking for a straightforward answer, trade with the Byzantine Empire helped Kiev Russia flourish. The Byzantine desire for slaves and forest products brought great trade between the two nations. Trade with the Byzantines also led to the conversion of Kiev Russia to Orthodox Christianity instead of adopting Catholicism, despite the fact many Russians would remain Pagan. There were very few major cities throughout the state and even Kiev, with a population between 30,000 to 50,000 inhabitants, paled in comparison to the other major metropolises of the time. Other cities only amounted to small fortified trading post, this in turn led to specialization in crafts such as glass making, a skill imported from Byzantium. Rulers would tax trade allowing them to gain large sums of money in order to support their armies allowing Kiev Russia to remain heavily defended from their enemies. Kiev Russia would continue to prosper until the Mongol invasion in 1237 which eventually did them in. Hoped this answer helped.