Which was the reason for the development of independent city-states in ancient greece

I think the question is why there were independent states and not a unified country, as in Rome for example or in other places?

The reason why there was no one big united Greek state but many smaller city-states is that Greek geography supported defense of smaller states and made conquest of them more difficult. This is because of the numerous mountains, islands and peninsulas.

The answer is society was a caste like system with poor whites at the bottom of the white social ladder.