Not only is the US population ten times the size of Canada's but the US has the largest single economy* in the world.
Since Canadians understand the US dollar and many Americans understand nothing else, it makes business sense to list on dotCOM which is visited by customers from all around the world.
If you are shopping on dotCA, you will see prices from dotCOM (or dotDE or dotUK or dotIE) in both the national currency and in loonies below that in smaller figures.
*The EU as a whole is slightly larger.
