One commentator put the finger on the problem: the direct election of Senators.

Senators were supposed to be the emissaries of State Governments, while Reps. were supposed to emanate from the people. This was thought to provide a useful tension. With direct election of senators, this tension is greatly diminished.

H'ever, direct election was allowed, even demanded, because of some serious scandals, including the Teapot Dome thing, involving the purchase of seats from legislatures.

Cunundrum.