This is because the US is a federation. It is a collection of equal, sovereign states. The Senate guarantees a body where every sovereign state has an equal voice.
The structure of our government is also one built to ensure that it's difficult to pass laws. One of the intended purposes of the Senate is as a check on mob rule. To prevent a simple majority from shoving things down the throat of everyone else.
The House ensures a law must have broad support among the people of the country, the Senate ensures a law must have broad support across the country.
Personally, I'm all for dividing up power like this and making it difficult to ram through laws.