Why were the ancient Greek city states not united?

Answer 1

They had been at war amongst themselves for hundreds of years, and the habit was hard to break. Each city had it's own objectives, and groups of them formed temporary alliances to achieve them, switching sides when there was a benefit to them.

To unite, they had to have a common enemy. When Philip of Macedon might have provided this rallying point, he had already bribed some cities to support him. Even within each city there were different factions willing to do virtually anything for self-interest. As Philip famously said 'there is no city I cannot capture as long as there is a path up which I can drive a donkey laden with gold'.

Answer 2

In ancient Greece, the people felt loyal to the cities in which they lived, and did not conceive of Greece as a united country. The lack of any efficient means of either travelling or communicating across long distances, made it difficult to get to know people in other cities or to create any kind of cooperative endeavor with them. Although there did exist a shared culture, that was not enough.

The city-states were isolated by mountains on the mainland, and by sea. There were vast stretches of sparsely populated country between the cities, and there were enough individual differences between cities to discourage unification. Only the threat of Persian invasion could bring the often warring cities together in a common cause. Eventually what united them into one nation was being conquered by Philip of Macedon.