Democracy in ancient Athens led to an increase in slavery, why?

Athens in the 5th to 4th century BCE had an extraordinary system of government: democracy. Under this system, all male citizens had equal political rights, freedom of speech, and the opportunity to participate directly in the political arena. Further, not only did citizens participate in a direct democracy whereby they themselves made the decisions by which they lived, but they also actively served in the institutions that governed them, and so they directly controlled all parts of the political process