My father was in the RCAF and was in the running to fly the Arrow if it was built (he did see prototypes). IIRC, he said the Americans wanted the designs destroyed because they feared the Canadians were not secure enough and so it might be stolen by the Soviets. It was during the coldest parts of the cold war after all.
This is the reason. One of my mentors was part of this effort; the Canadians were riddled with Soviet agents.
EDIT: downvoted for facts. Here's a reference:
But until then his assignment was to become Soboloff so convincingly that nobody would ever suspect he was a Soviet spy. Eventually, he was given responsibility for managing five recruited agents, including a Communist Party of Canada member from Toronto who worked for the company developing the Avro Arrow and provided its engineering schematics to the KGB.
An engineer actually smuggled out a set of plans out. And it only recently came to light. The plans were being exhibited in Winnipeg (I think) just before this whole covid thing happened.
For US aerospace contracts, one of the standard provisions is the disposal and destruction of any remaining products and tooling. This is done for compliance with ITAR regs, to prevent advanced technical information from reaching other countries. No buying a booster prototype at auction for you!
What was bizarre about (aboot) the Arrow program is that the entire company staff were fired, and the Canadian military was sent in to destroy all records, drawings, test data, and of course the destruction of the six flyable aircraft. The program was replaced with US made BOMARC missiles, so there's one clue.
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.
What on earth compelled them to order that? That's just so barbaric. As if they were trying to destroy evidence of a crime.
Surely it makes more sense to preserve them?! You might even try to sell or license the design to recover some of the $250m invested?