From 1787 until the passage of the 17th Amendment in 1913, Senate members were selected by State Legislators. This was a problem at the time because State Legislators were notoriously corrupt. Prospective members of the Senate could bribe their way in. In fact, at the time the Senate was often referred to as the "Millionaires Club".
The original framers of the Constitution originally wanted state governments to have some say in our National Government. However, by 1913 it was realized that a new form of electing senators was needed. Thus, the 17th Amendment was enacted.