Having only limited experience in this department, the guess I would hazard (for non-huge companies, at least) is that the costs (marketing, distribution in some cases, infrastructure, customer support, legal, compliance) for operating a business that serves the Canadian market just aren't offset by the market size, which is only about a 10th of the United States.

What I understand from my Canadian friends is that all things internet- and IP-related exist under a drastically different legal framework, so the legal and compliance cost issues are non-trivial. Also, I could see some of these differences having an impact on the actual business models of some companies listed.