Why Ancient Athens democracy was not really a democracy?

It is an academic question, but here a list of few (most important) reasons:

• Government of Athens was very often changed, by coup d`etat. Especially in times of the Peloponnesian War.
• There was plenty of political trials. Many opposition politicians was forced to leave Athens or killed based on very doubtful evidences prepared in politicaly-dependent courts.
• Meeting of all citizens (Ecclesia) was no more than a crowd (or mob) that was easy to control by some skillful demagogue: the number of people was to great for conducting typical responsibilities of parliament: creating law (legislation), and controlling the government or state administration. Few thousand of people couldn’t work effectively to conduct those responsibilities. Costs of communication between members of such a large group are to high.
• In real democratic country there is no need for institution like “ostracism”. Democratic system is too strong to have such “special protections” against ambitious politicians. Ostracism was the easy way to eliminate from politics those of Athenian politicians, who were dangerous for a people that had power, and ruled Athens.
• There was no strong, and stable “political parties” like plebeians, and patricians in Rome but many relatively unstable groups of interests built around charismatic leaders.
• Procedures of election to other Athens institutions (like Aeropagus, Heliaia), and their competencies was very often unclear (so there was a chance to political manipulations) or outright promoted some group of citizens (but I am not going to describe all their flaws because election procedures, and competencies were changed many times).
• City state of Athens was responsible for many war crimes - Athens soldiers many times were ordered to murder all men in a country (and to sell women, and children as a slaves). That kind of “holocaust” was a kind of standard in wars waged by Athens.
• And at last but not least: If Athens city-state had been really democratic, there would be a second empire like a Roman Empire in a eastern part of Mediterranean Sea. Athens would not loose war against Sparta. Real democratic system is too effective.

It is also good to compare some Athenian politicians (like Pericles) speeches with the ones of modern dictators for example Mussolini or Castro. They contains plenty rhetorical tricks used to turn listeners to speaker side, but a very few facts related to current problems of the country.

See also a short description of the Polity of Ancient Athens.