How did Ancient Greece transition from a kingship to city states?

To the best of my knowledge, everywhere else in the ancient world, like Egypt and Mesopotamia, kingship seemed to be far and away the most popular system of government. And in fact, ancient Greece had kingship, and was ruled by monarchs at one point too. But we know that the ancient states of Athens and Sparta transitioned away from kingship, a system of government I thought had been basically the only system of government in the ancient world. The process of Ancient Greece transitioning from kingship to city-states isn't something I've heard much about either.