Why didn't the silk road run north of the black sea?

The purpose of the Silk Road was to pass through as many commercial cities as possible. If the silk road ran north of the Black Sea, the traders would have encountered poor terrain, hostile nomads, and the route would not have many commercial cities. Remember that the Silk Road was an extremely long route to travel and only a handful of traders actually traveled the entire route. Most traders would drop off their goods with other merchants in the cities.