Question : coins - why only side is shown

in the US coins, why/what was the reasoning to keep the president's faces only by the side and not straight?

thanks

Answer : coins - why only side is shown

Some face front

http://www.google.com/images?q=2006+nickel&oe=utf-8&client=firefox-a&rlz=1R1GGLL_en___US376&um=1&ie=UTF-8&source=univ&ei=jwxOTNbtIYH-8Aaz_ZHyCw&sa=X&oi=image_result_group&ct=title&resnum=5&ved=0CEQQsAQwBA&biw=1188&bih=693