Why were the tyrants able to seize control on Athens?

Tyrants were appointed when there was a breakdown in the political system leading towards civil war. The function of the tyrant was to rule autocratically, but balancing the aspirations of the oligarchs on one hand and the lower orders on the other, to provide a fair compromise. Their problem was that when they had a fair system running for over a generation, the new generation forgot how bad things had been under the oligarchs, and expelled the tyrants for being oppressive, leaving themselves open to a return to the oligarchic oppression of earlier times.