Answer:
US population was too big.
Step-by-step explanation:
Democracy in ancient Athens allow the people to directly vote on he legislation that affected the whole countries.
Since United States have different regions that possess its own problems and opportunities, the decision that positively affected one state could be completely detrimental on the people that live in other states.
So, the framers of the constitution eventually decided to give each states with the power to regulated their own states rather than relying on simple majority decisions.