This was because the Peloponnesian peninsula is covered in islands and difficult to reach places without technology that didn't exist at the time. The city-states developed individually and did not share much with others since there was no centralized government for everyone and they were far away from each other. This is unlike areas like Egypt or Mesopotamia where everything was on a single piece of land that could be connected with roads and a central culture could easily develop and spread across the land.
It impacted politics in the mid-19th century because of the large inflows of immigrants after 1845 from cultures that were different from the existing American culture.
