4. Athens had a small population
The direct democracy work for the ancient Greek city of Athens because Athens had a small population.
The direct democracy (also known as pure democracy) is a form of democracy in which all citizens gather in one place and participate in decision making. They vote for all important decision to be taken. The majority decision is taken in account and decision were made according to it.
All the citizens (who have right to vote) were supposed to gather at one place. From this, we can say that this form of democracy will be practice in less populous states/district.
Athens, a city-states of ancient Greece, did not have a large population compared to the entire ancient empire. Such a form of democracy is not possible in modern time as we can't even imagine of gathering all the citizens at one place to discuss and make decision over political matters.
That's why, representative democracies are used in modern countries and decision is taken after discussion inside a places called parliament.