Athens in the fifth century BCE invented something genuinely radical: a public order in which decisions were not made by a king, a priest, or a hereditary elite, but by an assembly of citizens, voting directly. This was direct democracy. Every adult male citizen had a vote, and decisions were made in the agora, face to face, in deliberation that the entire assembly could hear.
The Athenian model had two structural limits, and these limits ultimately determined how far it could spread.
The first was a limit on participation. Athenian democracy included perhaps twenty percent of the city's adult population. Women, slaves, and metics - resident foreigners - had no vote. This was not a missed opportunity. It was a structural feature, given the technologies and assumptions available. The conceptual leap to universal participation would not be made for two thousand more years.
The second was a limit on scale. Direct democracy worked in a single polis with maybe thirty or forty thousand citizens, because all of them could physically gather, hear the same speakers, and vote in the same procedure. When Athens tried to extend the model to its allies, it quickly degenerated into imperial domination. Direct democracy at city scale was possible because everyone could be in the same room. At empire scale, the room did not exist. There was no technology to create one.
For nearly two thousand years after Athens, no one had the technology to overcome these two limits. So democracy as such largely disappeared from history, replaced by monarchy, empire, and oligarchy. Where it appeared - Venice, Florence, the Swiss cantons, medieval Novgorod - it was always small, local, and limited.