Alrighty, so let's talk Human Geography. What we're mainly going to be concerned with is population growth and migration with a little bit of political details. So you have this concept known as Demographic Transition which has 4 with a possible 5 stages that depicts population change across history and history generally but roughly adheres to this.

• Stage 1: Most of human history, you have high birth and death rates. Mainly infant deaths but you also have disease, famine, conflict, etc. High births and deaths cancel out. No countries are currently recorded to be in Stage 1

• Stage 2: Here is where population growth happens. The Industrial Revolution in Europe and North America coincides with high population growth. Advances in medicine bring down infant mortality and other cause of deaths while birth rates still remain high. Much of Sub-Saharan Africa and parts of South Asia are in Stage 2.

• Stage 3: Death rates are declining due to advances in medicinal technology but birth rates are falling faster from social advances. Women in the workplace, rising middle class, reproductive rights, etc. Much of Latin America, Southeast Asia, and the Middle East and North Africa are in Stage 3.

• Stage 4: Little to no population growth, developed countries like the US, Canada, Western Europe, Japan, South Korea, Australia, New Zealand, Argentina, and Chile are in this stage. Social changes have brought down birth rates and to around the level death rates are at. Life Expectancy is up. People know fairly reasonably their child will survive to adulthood and spend more on less children and contraception is accessible. Immigration is the major cause of population growth though this has proved controversial in developed nations.

Now Migration. It roughly works with demographic transition. Stage 1 has little migration except in times of extreme need. Stage 2 is emigration for land and jobs and other reasons. Stage 3 is immigration from declining birthrates and 4 is more of the same.

Now for the history:

• The US starts with a higher base to start with of around 4 million in 1790. There are no records from Canada at that time but the 1825 Census put you at around 480,000 to America's 10-11 million. Larger base population to multiply by I guess. I don't math good. Anyways, you have the English and African American populations to start, and large amounts of European immigrants arriving, as well in a time of large population growth.

• "Unrestricted" immigration from Europe. The 19th century has massive population growth in Europe and North America. Europe was 1/4th of the world's population in 1900. That share has declined due to growth in other regions. European emigration to the Americas is centered around the 19th and early 20th centuries. While many of our names sound English, many are somewhat Anglicized after time in the US. The largest ethnic groups of European descent are German at 17%, Irish at 12%, English at 9%, Hispanic Whites at 8%, and Italians at 6%.

• Sources Cont. So while Mexico is the largest all time source of immigrants, they arrived during the US period of Demographic Transition when birth rates were low. Germany is the second largest source and most arrived during Stage 2 so you have large growth and thus they make up a larger share of the population.

• Push and Pull Factors. Many immigrants were looking for jobs and land. Italian industry was concentrated in the North and still largely is so many Italians from the South come and settle in the Northeast. I'm part Italian and live in the Northeast actually. This is how the Mafia got here. Sicily was a major geographic point of origin. In addition some immigrants particularly in the 1800s though more early to mid were looking for land. While Canada has land for sure, it's largely concentrated in the north and is largely uninhabited. While a good amount of US land was uninhabited in our West, it was a bit more hospitable and arable in say Nebraska and Colorado than say Baffin Island.

• British Laws. Do remember that Canada is still part of the British Empire while the US is not. The UK was making their immigration laws and we made our own, they fit each country's specific needs. Britain was fairly obviously going to try to send in English or other British(Scottish & Welsh) settlers given that they have a lot of population growth and they have colonies they can funnel emigrants of their ethnicity and culture to. Even today a large percentage of Canadians are either of Anglo-Celtic origin or French origin. This is the same case in Australia and New Zealand minus the French obviously.

• US Laws. In the US we have a large influx of immigrants and while we had general preferences in that certain European groups were treated better, the Italians and Irish not so much, the US just wanted Europeans given that we passed the Chinese Exclusion Act to limit the sources of immigration. Though general anti Irish/Italian/Slavic sentiments would move along the passing of quota laws to limit immigration.

• So not only does the US have more to start, but we get more immigrants, we have more sources, and many come during a time of large population growth. While the US and Canada might grow by similar percents, the US has a larger group to grow with because of what was stated above.

Hope this helps. Woah this was long.

Wait so do you guys use MLA or Turabian or another style? I've mostly been required to use MLA in my writing.

Rubenstein, James M. "2 Population and Health, 3 Migration." Cultural Landscape. Harlow: Pearson Education Limited, 2014. 45-76. Print.

When the new world was first discovered and North America was being colonized, the English put many more people and resources into their colonies (What is now the US), while the French did not see as much profit to be made. Due to the climate of even the most southern parts of Canada, farming was impossible for about 8 months of the year (Believe me, I live in Canada) limiting food production and population growth.

When war broke out between the English colonies and New France during the seven years war, the French were vastly out manned and out gunned. As a result the English were able to take control of New France, Montreal being the last to fall (which is why they're still so French). When the US revolution occurred, there were some loyalists that moved to Canada which was loyal to the crown but not many. Now that I think of it this part is kind of irrelevant but I dont feel like deleting it.

TLDR: French did not invest as much in Canada as the British did the USA, and Canada's climate was not as ideal as her southern neighbor

Sources:

Anderson, Fred. Crucible of War: The Seven Years' War and the Fate of Empire in British North America, 1754–1766. Faber and Faber, 2001

Dull, Jonathan R. The French Navy and the Seven Years' War. University of Nebraska, 2005.

General knowledge from classes I had to take in school

Although I could give an answer, I wouldn't be able to accurately source it online, as my knowledge mostly revolves around schooling on the subject.

I will ask the OP to clarify, are you speaking in comparison to the American Colonies circa 1800, aka Pre-Confer, or are you talking about comparing it to America in the Post-Confed era?

This is a somewhat bizarre question, if I may say. Low in comparison to what? Canada is the 37th most populous country in the world, and much higher up if you remove second- and third-world nations. That puts it in the top fifth or so population-wise of all countries in the world, so this makes no sense.

Do you mean to ask why has its population density remained very low? I think that question is self-evident -- much of Canada is uninhabitable, and Canada is absolutely enormous. Otherwise, I simply don't understand the question.