It's a daisy chain of causes.
First, state legislatures are made up of fewer people who presumably are more well-versed on the issues. These are the people who would oversee the Senate, and likely pay closer attention. It's much easier to fool the people at large who are more focused on living paycheck to paycheck than paying attention to the politicians in Washington DC.
Second, the 17th amendment created new elections where there were none before. Today, that's 33 very expensive, and now mean and nasty, elections every 2 years. Senators now have to worry about campaign financing, when they didn’t have to before. The distraction of raising money takes them away from focusing on representing their states.
And third, the need to raise money causes special interest blocs to form. Candidates align around parties that represent agendas that interest groups are willing to fund. The parties themselves will centralize the interest group donations and disperse them to those candidates whom the party feels will be most effective in advancing the party's, and by extension the donor groups', interests. The candidates for office, and ultimately the incumbent Senators themselves, become beholden to the party's interests over the interests of the state that sent them, if they want that money to continue.
That's why it's important to repeal the 17th amendment and return control of the Senate to the states, because it breaks the money and party cycle by eliminating the elections altogether.
-PJ
What everyone ignores in these discussions of repealing the 17th is why it passed so easily to start with. To start with, a series of scandals had tainted the selection process, and senators were chosen not on merit, or on who would do the bidding of the state legislature, but on who the political machine in the state wanted to reward or, alternatively, who could bribe a small number of state legislators to swing the election.
On the other hand, political deadlocks in the states sometimes led to them failing to name a senator at all. Delaware went four years without a senator. Indiana had a deadlock that went two years. In all, there were 46 deadlocks between 1891 and 1905. In Oregon, in 1897, the minority third of the legislature refused to take the oath of office specifically to block the appointment of a senator, and, coincidentally, to conduct any business at all for a year.
The issue of who the legislature would name as senator came to dominate the state elections, as can be seen in the Lincoln-Douglas debates, with the men campaigning up and down the state for a seat that the legislature, not the voters directly, would choose. The state legislators were reduced, essentially, to senate electors who incidentally also sat in their legislature. Certainly the state issues took second place in those elections.
Amemdments to go to direct elections had been proposed as early as 1826 and passed, and by 1910, thirty-three states had enacted some form of direct election on their own, in the form of party primaries. A few had gone as far as enacting direct elections and more were preparing to do so.
Once submitted to the states, the 17th sailed to easy ratification.
Thanks for the best explanation I’ve heard on this issue.