No need to ask...read the interview with the man himself and so have others on this forum.
One of the engineers responsible for the design of H-D's 6 speed made the same comment I posted re strength. He was interviewed a while back in American Iron Magazine I believe. They chose straight cut for 1st gear to handle heavy loads and the high torque multiplication, needed the same for 5th cruising under load.
Straight cut was more efficient, cooler and stronger in this application.