The US-Canadian border is the longest land border in the world. Canada and the United States are two developed countries, and the US is one of the largest economies in the world, in addition to being the largest economy in the world for a good portion of the 20th and 21st centuries.
As with every other type of business, this means that a market can easily be created for any type of illegal activity in both countries, but especially the United States due to it’s large size. The fact that Canada is nearly 10 million square kilometers, yet has a small population, means that policing the borders will largely become a function of the US, due to having more manpower.
The US population will also grow much faster than Canada’s, but Canada will potentially grow as a market as well, as it’s population increases yet it’s income, economic power and standard of living remain stable.
It would literally cost trillions of dollars for the US and Canadian governments to proactively police the borders. A wall between the two countries will be less beneficial in the long term than if there were open borders, as the legitimate industries will benefit multiple times over illegal industries, as legitimate enterprises account for most of the economy in both countries anyways.