There were good reasons for the amendment. Basically the senate was in the hands of the various states' political machines. There were a number of bribery scandals--it's easy to buy a senate seat when you only need to swing a handful of votes in a state house. And by the time the amendment passed, the majority of states, on their own, had already gone to some form of a direct election system.
Why even have 2 legislative bodies if both a popularly elected? The Founders knew what they were doing. The U.S. was to be a federation of sovereign States. That's why they gave tiny States 2 Senators, the same as much more populous States. Indirect election was an important part of Federalism (now almost dead), and part of the reason we had a republic, not a democracy.