Because the primary threat identified by American counterintelligence, correctly, was the cost of the Soviet Union taking the Canadian work and running with it

Because the primary threat identified by American counterintelligence, correctly, was the cost of the Soviet Union taking the Canadian work and running with it (See, among others, agent LIND in the Mitrokhin Archive) was greater than any defense gain from the airplane. It is a backhanded compliment that the material was destroyed.