I'm not asking why did they not have a king instead, but rather why the choice of words.
Was it more popular? Was it because it wasn't regal and powerful sounding? Did that mean the title had a certain connotation? Or did they just go for literal sense of someone presiding?
Both Houses of Congress appointed committees to consider the proper title to give the President, but they could not agree. The Senate wished it to be "His Highness the President of the United States of America and Protector of their Liberties." The House considered this as too monarchical, and on May 5 addressed its reply to the inaugural speech merely to "The President of the United States." The Senate on May 14 agreed to this simple form.
http://www.archives.gov/exhibits/charters/print_friendly.html?page=constitution_q_and_a_content.html