What kept greece from unifying under one political system

There were many reasons for not unifying in one political system:
One of them was the geography, Greece contained many mountainous terrains, valleys which were separated from each other, many islands where it was easy to form new towns and settlement, giving birth to many power centers instead of one
Another reason was the sense of heroism in Greece and to fulfil that the great Mediterranean, they could sail to wherever they wanted in the times of need resulting from any kind of crisis and this helped them establish new colonies all over the Mediterranean.
Third reason was the presence of aristocracy who denied the establishment of a monarch and each city or state defended their independence.