Answer: Because all the members of the single house are elected by the people.
Context/detail:
The Articles of Confederation, which preceded the creating of the Constitution of the United States, employed a unicameral (single house) legislature. In the end, the United States Constitutional Convention in 1787 decided to go with a bicameral (two-house) legislature as a compromise between small states vs. large states in regard to representation. "The Great Compromise," as it became known, resolved a dispute between small population states and large population states. The large population states wanted representation in Congress to be based on a state's population size. The smaller states feared this would lead to unchecked dominance by the big states; they wanted all states to receive the same amount of representation. The Great Compromise created a bicameral (two-chamber) legislature. Representation in the House of Representatives would be based on population. In the Senate, all states would have the same amount of representation, by two Senators. In the original US Constitution, each state's senators were selected by that state's legislature, not directly elected by the people. The 17th Amendment changed that, so that now US Senators are also directly elected by the people. But we retain the two-house legislature as a way of balancing power between states with high populations and those with lower populations.
