In short - Geography.

I think it is easier to see one of the underlying reasons for the disparity in population by visually comparing geographic regions. About a third of the country's land mass is primarily composed of igneous rock formations dotted by literally millions of lakes, swamps and forests. It is very expensive terrain to build permanent infrastructure on, and it generally lacks agricultural value. Transportation is, therefore, very expensive, and the economic base of the region rests with primary industries like mining and lumber production.

West of the Shield there are the northernmost extension of the Great Plains, who are also the lower 48 states most sparsely populated regions besides the mountainous west - which Canada also shares. The west coast of Canada is very rocky and involves slightly more navigation as well - so port capacity is very low by American standards. In fact, the entire country has far less port capacity as a whole due to geography than its American counterparts.

So the bulk of the country's economic activity occurs in the St. Lawrence river valley, and the fertile lands of southern Ontario. Population densities in these areas are very similar to population densities across the border in similar geographic areas with similar economic comparative advantages. Economic activity generally attracts the populace.

Actually, if you compare some Canadian geographic regions with their American counter parts (ie: Great Plains), you will find that the Canadian component of those regions actually has a higher population density. IT's just that the country as a whole has major geographic disadvantages when it comes to economic output that her American neighbour is unhindered by.