I'll give you an example: the two U.S. Senators supposedly from Michigan, really are from Detroit and its immediate suburbs. The rest of the State is not represented - Michigan as a whole, is not merely Detroit.
By mandating popular election of Senators, the power of the urban areas made a dramatic jump. By returning to a state legislature election, the outstate interests might just get a hearing.
It's been awhile (perhaps too long) since I read the Federalist Papers, but one of the notions underlying the system of government was that as long as different factions were fighting with each other to maintain their own powers, liberty would be safe. The danger would be if any particular faction was too successful in concentrating power unto itself.
If you were a state legislator, would you support a Senator who threatened to withhold highway funds from states that didn't impose a 0.08 BAC level? Or would you see such a move as a threat to your own personal power, even if your state happened to have a BAC of 0.08 already?
One of the major principles of federalism is that different people in different places get to live under different laws. If 67% of the people in 67% of the states want to live under a certain law, but only 33% of the people in the other states wants to, should everyone have to live under that law?
As I've drawn up the numbers, 56% of the people want such a law. So imposing it nationwide would make 56% of the people happy. On the other hand, if the law were imposed only in those states that wanted it, 67% of the people would be happy. So which is a better solution?
If people elect Senators directly, it's likely that 67% of the Senators will support imposing the law nationwide. If legislators elect Senators, however, it's much more likely that the legislators in states where such a law is desired would impose the law themselves and not pressure their Senator to do so. After all, they've given their people what they want without involving the Senate, and thus avoid giving away any power they don't have to.