@valleyforge As I recall, it was a response to the perceived corruption in that the Senators often ended up being cronies of the state legislators, or at least people to whom the legislators owed some favours. In theory, the Senators were indirectly answerable to the people since a state legislator would be punished by the voters for electing an idiot; in practice, the electorate proved unwilling to hold poor performance of a Senator against the legislature that elected him.
@valleyforge I don't think voters hold senators responsible for the justice's behaviour, really. If a Senator votes for his party's nominee, nobody gives him a hard time about it - it's only when he shows disloyalty by failing to support somebody that he might give ammunition to a rival in the primary.
For instance, I see a lot of conservatives trashing Chief Justice Roberts lately. But nobody's seriously going back to his confirmation hearings and using that as an excuse to ditch the Senators who approved him (quite a few still hold their position, actually).