In the years before Cleisthenes' was born, the most influential man in the city had been Solon, an unselfish and model aristocratic reformer who became known as one of the Seven Wise Men of Greece. Solon had sought to limit the excessive powers of the nobility and restore Athens to a state of 'eunomia', or 'good order'. To achieve this he had created a Council of Four Hundred men whose job was to represent the population as a whole, and encouraged the people, especially the aristocrats, to be responsible for their city, not just themselves.
Solon's ideas were, however, ahead of their time and the ordinary people still had little political influence. Their main role in the politics of the city was to act as the supporters of their aristocratic leaders, not act for themselves.
So when Cleisthenes was only 10 years old and his brother-in-law Pisistratus, a popular general, seized power there was little opposition from the ordinary Athenians. After several failed attempts, Pisistratus ultimately established himself as 'tyrant', the undisputed leader of the city.
Who was Solon?
How Pisistratus took power
Pisistratus Rules as Tyrant & Reforms The Economy