If I could ask a related question, why is Siberia so much less populous than China?

To add to your question in case someone can answer, I wonder what factor slavery had the population difference?

Consider a phisiographic map of North America. Here http://www.roebuckclasses.com/105/regions/namer/namericaphys/physnamer.htm

The areas most suitable for agriculture, one of the first European economic activities, are basically the plains and interior lowlands with some coastal plain.

Those areas of Canada which qualify in Canada all have 18th century issues. Coastal plains are minimal and Nova Scotia was a war zone in any event. The Great Plains where simply inaccessible. The British were venturing into Hudson's Bay, but on a strictly mercantile basis, think fur trade, and were in any event cut off by the Canadian Shield.

The main access, up the St Lawrence river, was blocked by a rival foreign power in the form of Quebec. This was the case until the end of the 7 Years War in 1763.

Once Britain owned Quebec this impediment was removed and settlement in present-day Ontario opened. The initial influx was Loyalists, Americans who chose to remain loyal to the Crown, who received grants of land and other aid from about 1780 onward.

The main settlement of the Canadian Plains waited for a trans-continental railway on a Canadian route. So Canada had a climatic and topographic handicap.

Source Canada: A Regional Analysis Putnam, D.F. & Putnam, R.G.