From Wikipedia 12 hour clock
The National Maritime Museum, Greenwich, states:
To avoid confusion, the correct designation for twelve o'clock is 12 noon or 12 midnight. Alternatively, the twenty-four-hour-clock system may be used. The abbreviation a.m. stands for ante-meridiem (before the Sun has crossed the line) and p.m. for post-meridiem (after the Sun has crossed the line). At 12 noon the Sun is at its highest point in the sky and directly over the meridian. It is therefore neither "ante-" nor "post-".[9]
Maybe this is also part of the reason. The US just doesn't want to change things. 
According to the CIA Factbook only Burma (Myanmar), Liberia, and the United States have yet to adopt the International System of Units as their official system of measurement.[
