Why were sumerian communities were called city-states?

Each of the cities were a separate political unit. As also with the ancient Greek cities states like Athens, Sparta, Corinth, etc. and the Italian cities during the Renaissance like Florence, Venice, etc. The city owned the land around it. People owed their allegiance to their individual cities. Even today in the time of national states, we still use words like politics, political, politician which stem from the word "polis" meaning city.

the exclusive possession or control of the supply or trade in a commodity or service.