Which factor prevented the Mesopotamian city-states from uniting before 2400 BCE?

Before about 2400 BCE, Mesopotamia was divided among independent city-states that competed with one another. None had a ruler powerful enough to conquer and control the others; later, Sargon of Akkad built an empire that united many of them. The city-states were close enough to interact and shared important cultural features, so distance or major differences in religion and language were not the main barrier.

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