The impetus for the direct election of senators came from the political corruption endemic in the post-Civil War era. Corporations "owned" towns, counties and even entire states. As a result the senator from California was often referred to as the "senator from the Southern Pacific Railroad". The Progressive Movement wanted to purge the system of corruption with the following reforms.
The last item was a popular cause of the day. The 17th Amendment easily passed the House, but the Senate always killed it. Then state after state requested a Convention for Proposing Amendments to address this issue. Once the state-count got to within 1 or 2 states of requiring a convention call, the Senate buckled. The states ratified it in very short order.
Thanks for that bit of history, Publius.