Why didnt oligarchy last?

The oligarchy in Athens was replaced by a tyranny set up to give fair treatment to the lower class citizens against oligarchic rule. When the tyrants were expelled, the oligarchs tried to regain control but were preempted by Cleisthenes setting up a limited democracy. When the Persian invasion began the oligarchs regained power to organise the war effort, and hung on to it afterwards until Ephialties re-established the democracy, He was murdered for this but his deputy Pericles kept it going.