Geography and technology. The conditions to support large cities were not possible with the technology of the day.

• Climatic conditions generally favor agriculture in warm, oceanic climates with mild winters for early civilizations (more food variety, higher yield). Cooler periods hit places in the far north harder than the Mediterranean. The early rise of the Rus cities coincide very roughly with the Medieval Warm Period (~950 - 1250)
• The soils of Eastern Europe are often either less fertile (Podsolic) or need significant irrigation infrastructure (Chernozem) for its high yields of today. Some of them are harder to plow and could only be farmed with further advances in plow technology. Horses were not initially used due to the lack of suitable horse collars, while Oxen were less suited for northern climates. Cities generally need a lot of excess food to feed their population.
• Distance: Cities rely on trade routes (e.g. getting grain from Egypt to Rome or Constantinople). Transport by sea or river was, in the past, by far the fastest way to transport goods. The White Sea was too far north for largescale trade. The Baltic was used (e.g. by Novgorod). The Volga traderoute / Northern Arc was likely only established around the 8th century.
• Terrain and defense: Close distance to pastoral nomads (Scythians, Huns, Mongols) often meant conflict. Less natural defenses and flat terrain led to a bigger advantage for mobile nomadic cavalry.

Geography and technology. The conditions to support large cities were not possible with the technology of the day.

• Climatic conditions generally favor agriculture in warm, oceanic climates with mild winters for early civilizations (more food variety, higher yield). Cooler periods hit places in the far north harder than the Mediterranean. The early rise of the Rus cities coincides very roughly with the Medieval Warm Period (~950 - 1250)
• The soils of Eastern Europe are often either less fertile (Podsolic) or need significant irrigation infrastructure (Chernozem) for its high yields of today. Some of them are harder to plow and could only be farmed with further advances in plow technology. Horses were not initially used due to the lack of suitable horse collars, while Oxen were less suited for northern climates. Cities generally need a lot of excess food to feed their population.
• Distance: Cities rely on trade routes (e.g. getting grain from Egypt to Rome or Constantinople). Transport by sea or river was, in the past, by far the fastest way to transport goods. The White Sea was too far north for largescale trade. The Baltic was used (e.g. by Novgorod). The Volga traderoute / Northern Arc was likely only established around the 8th century.
• Terrain and defense: Close distance to pastoral nomads (Scythians, Huns, Mongols) often meant conflict. Less natural defenses and flat terrain led to a bigger advantage for mobile nomadic cavalry.