This answer by /u/bigslothonmyface explains why "President" was chosen over a more elevated title. They don't give a reason why "President" specifically was chosen though.

I remember a historian commenting on this on a radio program some years back. He was discussing how the meaning of words change.

He said they chose they wanted to least grandiose title they could find (opposite of a King for reasons...) and settled on title of President as it was normally used for the leader or chairperson of a small meeting at the time. That was sufficiently boring so they went with it.

It became grander afterwards defeating the purpose.

Here is a similar NPR article.

https://www.npr.org/2016/02/15/466848438/why-president-how-the-u-s-named-its-leader