Because it turned the Senate into a second House. Prior to the 17th, senators were selected by the state they were from, not directly elected by the people of that state. This process varied by state some. States either had the legislature select the state's senators subject to approval by the governor, or the governor selected them subject to approval by the legislature. No election involved. State government selected their senators to represent the State, not the people. The People were represented by their representatives in the House.
With the 17th, the people selected their senators just like their representatives and defacto turned the Senate into a second House. The states were no longer represented.
The reason this was done by the founders was to further divide power among more different and disparate groups so none had too much.
At least you asked. All evince can do is rant and rave with profanity at her complete lack of knowledge on the subject preventing even a scintilla of rational response.