Several reasons:
1 - The obvious one: the climate. Champlain had to retry 3 times to get a pair of colonists who survived their first winter. Except for Vancouver, even our mildest winters are "Minnesota winters" (or harsher).
2 - Soils: Pretty much the entirety of Canada was glaciated, as a result land easily suited for agriculture is scarce. Most of the territory is too rugged, soil-less, covered in boulders or underlain by swampy muskeg. Good agricultural areas also happen to be clustered in the south along the St-Laurence seaway, the great lakes, the coastal Maritimes and the Okanagan valley.
3 - Access: Canada is crossed by a mindboggling number of rivers, which complicates (and at times precludes) building roads.
4 - "Repulsiveness" -all of the above elements combined to make Canada a somewhat less attractive destination for prospective immigrants looking for land ownership in an agrarian society, and as such the rate of immigration to Canada was never remotely close to that of the US, which really affected the demography.
hi! you may be interested in some previous responses