Why didn't the oligarchy last?

Answer 1

Each city-state had its own form of government. Narrow-based oligarchies - rule by an aristocracy - were generally replaced by broad-based oligarchies, which were usually comprised of either a broader council of several hundred or often several thousand in an assembly comprising landowning citizens who also made up the armoured infantry on which the security of the city-state depended.

Starting with Athens in the 5th Century BCE, an experiment in democracy (literally people-power) started to spread to other cities, but this was adversely affected by Athens which turned the anti-Persian League into an empire of its own, and bringing on the devastating Peloponnesian War. Cities which had embraced democracy reverted to broadly-based oligarchies.

After this, the radical democracies of Athens and its copyists, where all decisions were taken in the fortnightly citizen assemblies and the Council carried out its decisions, they generally were watered down to the citizens voting on motions put to them by the Council. And attendance at the Assemblies declined, so that the Council had to offer inducements to get them to attend the city's Assembly. The gloss and novelty of the early period had worn off quickly, and the mistakes made by the democracies had made for a drop off in popular enthusiasm which was substantially replaced by apathy, into which void the influentials moved, and resumed governing with the people asked to confirm laws and policy already decided on.

Even during the high period in Athens of fifty years, in emergencies the oligarchs or the landowner/warrior class took over to solve the problem rather than leave it to endless debate in the Assembly, using a simple takeover, or calling assembly meeting in the countryside when it was insecure due to enemy besiegers, so that only those who could afford a suit of armour would dare attend.

So broadly based oligarchies continued to exist, and democracies became pale shadows of the real thing which Athens had pioneered. After Macedonia had become dominant and Alexander died, the Greek world became dominated by his successors the Hellenistic kings in eastern Europe and Asia, and in the central Mediterranean tyrants and kings had established themselves well before this ruling the Greek cities there.

Answer 2

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.