Just imagine how much money would be taken out of the "campaign trail" if Senators came from the ranks of a state legislature. They would no longer need to appeal directly to the people and a simple letter to your local representative would be sufficient to show who you support.
That reason alone will prevent it's repeal from ever happening, too many rich elites who buy senate seats through campaign financing.
What you didn't see was guys like Jon Corzine, Herb Kohl, and Mark Dayton; guys who had this vast fortune and wanted a Senate seat for personal gratification. In order to become a senator back then, you had to have a track record either in the state legislature or as a civic leader.
Supposedly, the six year term would provide some protection from unpopular decisions. Now the senate is nothing more that another branch of the House of Representatives with richer people.
But once they have to bribe a bunch of local yokels to get the gig, the cost may actually go up.