Why did the highly developed democracy in Athens fail in history?

The history students are here.

Do you remember? Athens has democracy and takes turns to rule. Although taking turns to govern is a form of democracy, its biggest limitation is that it enables all kinds of people to come to power, including some people with low quality and poor ability. This led to the decline of democracy.

Even in a highly democratic country, there is no such policy of taking turns to govern. .....

It has nothing to do with productivity level, thank you.