The Congress is also split the way it is due to historical reasons. The House and Senate are meant to mimic the House of Commons and House of Lords in the sense that one chamber represents the "regular people" and the other represents the "higher interests", although this purpose itself isn't written into the law, but was historically treated that way (James Madison, however, did distinctly state that the Senate needs to exist so that the rich won't get their money voted away from them).
They're also set up to represent both the people and the states as two separate groups, because while the states more or less act as glorified provinces today, during the earliest years of the US they were legitimately sovereign states that were simply in an alliance with each other like the EU. The original founding document, the Articles of Confederation, floundered and they needed to establish a stronger federal government through the Constitution, but there were debates between the big-population states and the small-population states about how to properly allocate votes. So they compromised and said that a bicameral legislature, in which one chamber's representatives would be elected in direct proportion to the people and in which one chamber's representatives would be assigned for each state, would be put into effect. This way the states were equal to each other (all states having two senators) while the democratic element got its way as well (proportional representation - this is also why tax issues have to come from the House and not the Senate, since consent for taxation comes from the people.). Initially, senators were even chosen by the state legislatures themselves rather than voted on directly by the state populaces.
It's quite a complex system and it has both merits and flaws.
