Blame Canadian content laws. Amazon is classified as a bookstore in Canada, and therefore must sell X percent Canadian content - a protectionist policy that supposedly protects us from evil Americans taking over our media. The net result is that Amazon could either stock and attempt to sell a boatload of Canadiana which sell poorly (and in Amazon's low-margin line of business, is death), or Amazon can choose not to operate in Canada at all. It has chosen the latter. Amazon.ca is operated entirely from within the USA (no employees or warehouses in Canada), and all shipping is contracted out to a 3rd party (which, being a shipper, is not subject to the content laws).
Funny enough though, the shipper that handles all of Amazon.ca's work, the one that helps the company dodge Canadian law - is none other than Canada Post, owned and operated by our government. Some things just don't make sense.