Why does Canada have a low population?

 This.

Very pretty, but it doesn’t grow potatoes.
Most of the Canadian land mass in eastern and central Canada is dominated by the Canadian Shield.
If you are lucky, you get a centimetre or two of thin, highly acidic soil.
Historically, populations only grow along routes of easy transport with arable land. You only live where you can either grow your food or get it transported easily to you.

Here is a map of Canada’s population density:

Here is a map of the Canadian Shield:

Note the congruency?