When you buy a book in Canada, whether it is at a physical store or online, you are more than likely going to be enriching one company, Indigo. Indigo currently accounts for nearly 67% (44% excluding online and mail-order) of all national sales. The largest book seller in the United States is B&N with a 15% market share.
I have nothing against Indigo, obviously they must be doing a lot of good things to have such a large market share but if you are an author trying to sell your book in Canada you better hope an Indigo buyer likes your product. If Indigo declines you just lost half the market to sell your book.