The picture on each U.S bill is decided by the Secretary of the Treasury. This was established back in the year 1862 and has not been changed since. Each president was chosen as a result of the decisions made by the Treasury and no specific reason was given.
The Secretary of Treasury decides, its been like that since 1862 and hasn't been changed since. Not all the people on our bills were president, Benjamin Franklin wasn't but he's on the $100 bill. Not all the people got on for important reasoning one and Five dollar bills were decided by a coin flip. They do not have to give any specific reasoning for their choice.