Why did congress place the house and senate leaders at the top of the list?

I think the answer to your question has a lot to do with the relationship between the President and Congress. Congress sees itself, appropriately, as the executor of the people's will, and a check on the President's execution of their demands (laws). the President and his administration do all they do at the pleasure and consent of Congress, not on a day-to-day basis, but through legislative action. if Congress had not created all the various cabinet departments AND funded them, the President would do only those things laid out in the Constitution - for example, sign bills into law, veto bills,