The question is more geography than history. Canada and the US are vastly different climate zones. As such roughly 80% of the Canadian population is within 100 miles of the US border. Too far north you cant get annual agriculture needed to sustain large populations. Only a small portion of the Great Plains extend into Canada. Most of Canada remains wilderness with forests, mountains, and glacier lakes. Above a certain latitude you get tundra. There's certainly resources to be extracted in these areas, but food and supplies have to be shipped there to support them, and people as well.

A bigger factor though, the interior of the eastern US is full of the Mississippi River and its tributaries - easily navigable water sources. Meanwhile the gulf coast and Southern eastern seaboard is protected by barrier islands. This provides an exterior navigable waterway along the coast. New Orleans connects the Mississippi waterways to this coastal waterway. Additionally the US has plenty of natural harbors. Meanwhile the Great Lakes locks and canals system making those waterways navigable weren't completed until 1959, and still mostly serves as a northern connection to this interior waterway network. Today you can navigate by boat from the great lakes, down the Mississippi, up around the coast and back into the Great Lakes. Canada is really only involved for less than a quarter of that trip.

It cannot be understated how important navigable waterways and coastlines are for population and commerce, and agriculture. For most of human history, and to this day, water transport remains the most efficient method of mass transport. In terms of population density, you'll find waterways determine the majority of settlements. Even interior areas of the US are less populated the further away from navigable waterways. Canada mostly lacks these waterways, same for Mexico. It's positively absurd how advantageous the waterway system is for the US.

Now historically, possibly Canada could have expanded into the Mississippi Basin before the US, so that's where history comes in. The Seven Years War was caused in part by American colonists expanding into French and Indian Territory because it was there, and the French were mostly fur traders and trappers instead of permanent homesteaders. Many of the expansionist tendencies were kept in check by British rule, and those tendencies were more encouraged by American political leaders. This desire for expansion was partly due to better survivable conditions that lead to more immigration and great population growth. So population growth and expansion followed the geographical and technological capacity.