Answer:
Because it has two houses.
Historical context:
The Legislative Branch of the United States was created as a two-chamber ("bicameral") institution at the Constitutional Convention in 1787. There was initially 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. What became known as "the Great Compromise" created a bicameral (two-chamber) legislature. Representation in the House of Representatives would be based on population. In the Senate (the other chamber of the legislature), all states would have the same amount of representation, by two Senators.