The best explanation as to why direct democracy would not work well in the United States is that

voting is not mandatory in the United States.
the use of direct democracy would make debate on issues by individuals too difficult.
Ancient Greece was smaller and less geographically diverse than the United States.
the population of the United States is too large.

the population of the United States is too large.

The best explanation as to why direct democracy would not work well in the United States is that the population of the United States is too large. Direct democracy involves the participation of all citizens in decision-making, where they vote directly on policies and laws. In a large and diverse country like the United States, it would be logistically challenging and time-consuming to gather the opinions and votes of such a large population on every issue. Additionally, managing the logistics of organizing and counting votes across such a vast and geographically diverse nation would be a significant challenge. Therefore, the sheer size of the United States population makes direct democracy less practical for effective governance compared to representative democracy, where elected officials make decisions on behalf of the people.