I have the oportunity to speak to several retail store owners and factory reps about this issue. In addition to the blame the military excuse which though sounding like bull####, I have heard another exuse!
The way it used to be! There was a standing deal between all major suppliers in US (Rem, CCI, IMR, Hogdon, all the biggies, you get the idea) that they would send as much as needed to Canadian suppliers and would keep sending til need was met.
The way it is now! The US now requires a permit for each and every order no matter how large or small and a hugh pile of red tape that has to be cut to get the stuff accoss the border. A case of primers or a truck load requ1re the same amount of red tape. Gun parts require more red tape than guns. With all the hoops that a retailer has to go through, one large order is simpler than many smaller orders. Large retailers have to put up with the unpleasant task of dealing with pissed off customers. Smaller retailers and smith simply can not afford to jump through the hoops and go out of business.
George W personally does not horde that much ammo on his ranch, but he is directly responsible for the shortage of large rifle primers for sale in Canada.