Why is Marine one called Marine one?

The Marine Corps was assigned the sole responsibility of providing helicopter support to the President, worldwide in 1966. The Presidential helicopter assumes the call-sign Marine Oneonly when in possession of the President; just as the President's 747 (operated by the U.S. Air Force) assumes the call-sign Air Force One when the President is on board.

This is done so that when Marine One or Air force One are dealing with air-traffic control, there is no ambiguity as to the importance of the flight in question. Air force One and Marine One are always given priority over other traffic.