Direct democracy worked in Athens but not in the United States primarily due to the differences in the scale and structure of the societies. In ancient Athens, the city-state was small, allowing citizens to gather in person to discuss and vote on policies directly. In contrast, the United States is a large and diverse nation with a significant population, making direct participation in government impractical. Thus, representative democracy, where elected officials make decisions on behalf of citizens, was established in the U.S. to manage these complexities.
Given your options, the best rationale would relate to the size and structure of the population. The United States has a much larger population that makes direct democracy unfeasible, whereas Athens had a smaller, more manageable number of citizens actively participating in governance.