What happened here...as the turn of the century occurred is a couple of news magazines started to appear that brought a centralized news function to the general public. You could subscribe and get a view of things that went beyond your local area, your city, and your state.
So at some point, the news magazines focused on the corrupt nature that states were using to get individuals through the state legislative process and to the point of US Senator. Deals were made. Some corruption charges were made up in various states, and this got national publicity compliments of the news magazines. Prior to this period, there was simply no way that you got a national view of politics or national corruption.
So the agenda pushed forward, why not dump the legislative process out of this and just make people run in the state with regular voters selecting them. This was a GOP theme at the time...to clean up state politics.
The Amendment was proposed in 1912, and stepped through the process, and was effective by the summer of 1914.
Amusingly enough today, people feel the current system is corrupted and want to reverse it and return to the legislative process that existed prior to 1914. My guess is that it’d take less than ten years for people to admit that changing back to the old way simply re-created the whole corruption factor.
Perhaps the better method here is to limit each state to one single Senator, take away most legislative power, serve only to approve cabinet officers, impeachment processes, and approve Supreme Court members.
I addressed the Progressives over-hyped corruption in my two previous squibs.