Because Boeing wanted it that way and Airbus didn't.
The 727 also has upper and lower rudders. Both the 727 and 747 have yaw dampers on both. On the 727 the lower rudder has more throw when the flaps are not up. On the 747 the rudder ratio control varys the throw of both rudders as the speed changs.
Last edited by MarkerInbound; 7th Aug 2012 at 00:25.