Going back to your point about Purolator, remember that it is like the other courier companies: It really only serves the urban and suburban markets, so it's able to cherry-pick. They also charge a higher cost. It's $1.44 to mail a letter in Canada. It's not $1.44 to ship something by Purolator.
That business has taken the high end of the market, and it competes very successfully against the other courier companies. Canada Post offers a different kind of service.