The article doesn't get into how Soviet spies were uncovered in Canada in the 1950s and 60s. Governments were not being paranoid in the face of those revelations.
Right, the spy threat would be the "security reasons" I guessed at.
But still, wouldn't successful projects which were later decommissioned be more at risk of spies than an unsuccessful project? Yet successful projects do not have their blueprints and airframes routinely destroyed without a trace.
