When the country was founded they needed to convince the state governments of a bunch of different states to agree to unify as a single nation.

At that time state governments had more power and influence on the federal government than they do today. Senators were picked by the state government, not popularly elected as they are today, and it was left to the state governments to decide how they would conduct elections and select delegates to represent them in the Electoral College which is the actual body that picks the president.