The electorate had their own interests more closely represented, by district, apportioned by population, through their Congressmen. That is why Congressmen have shorter terms than Senators so that the electorate could more rapidly address their ever changing wishes in Congress. Senators were presumed to be chosen for their abilities by a more stable and better informed body than the electorate, the state's legislature. That is what they were hired to do by the electorate. Manage the interests of the state.
Now Senators are hired by the electorate, two per state, no apportioning, and go off to D.C. for six years where they entrench themselves in a self-protecting bureaucracy where they are unanswerable to state officials by censure or recall and nearly unanswerable to the electorate having found an unlimited trough of re-election funds and a good-old-boy re-election support network.
The Senate was to be a check on the House. The House, working directly for the people, would craft laws and treaties and bills. The Senate (which could also do those things) would represent the individual state's interests. The Senate, being longer tenured and more narrowly chosen, was to be the august tempering body that kept the more mercurial, temporal and widely dispersed interests of the House in check.
The state loses power to the feds because they must now grovel at a Congress's feet for authority and money. (One is usually tied to the other it seems.) So now both Senators and Congressmen are elected by popular vote which is fickle, being relatively easily manipulated in comparison to state officials, and self interested rather than interested in the state's sovereignty and welfare.
The 17th Amendment was a bill of goods sold to the people with the basic slogan of the Dems after election 2000; "the people are being disenfranchised." It was a way for the feds to consolidate more power and the people who pushed for it knew that. They were well aware of the maxim of pure democracies, to paraphrase; "once the people find that they can vote themselves largesse from the government they will do so until it is bankrupt." For wannabe petty tyrants that's a great thing. The more instability there is the more power they can assume. It was a major and necessary step towards socialism and the eventual dictatorship that we will have.
Next step: Woodrow Wilson and the League of Nations.
Next step: FDR and Supreme Court supremacy.
And don't forget about the Senate's advise and consent responsibilities. The Senate is responsible for confirming Executive appointments.
-PJ