The Seventeenth Amendment to the US Constitution changed the method by which Senators were selected. Bill Quick provides the simplest, best explanation for why the Founders' original method of populating the Senate was the wisest:
"There is no leash on the Senate. Originally, it was really supposed to represent the interests of the individual states, but the Wilsonian progressives managed to destroy that aspect of the body so, in fact, it doesn’t represent much of anybody. Senators serve longer terms than members of any other branch. Originally, this was intended to make the states, which appointed Senators, the most powerful force in the federal governance. America, after all, started out to be a union of states, not of people. And the self-interest of individual states worked to assure that the Senate wouldn’t go too far off the rails, because state pols were much less susceptible to political demagoguery than were individual voters. Piss off your governor and your statehouse, and you weren’t long for your cushy federal senate sinecure. All that is gone now. "~ From his post "History Rhymes".