> Because Canada is just part of the U.S.
As a Canadian, I'm amused to hear this because it is basically true as a first approximation.
Random factoid - Canadian coins ($2, $1, $0.25, $0.10, $0.05, $0.01 (withdrawn)) come in almost the same denominations as US coins ($1 (uncommon), $0.05 (rare), $0.25, $0.10, $0.05, $0.01), and they are the same diameter and thickness, but maybe having different weight and magnetic properties. It's kind of scary that Canadian coins are essentially state-sanctioned counterfeits of US coins.
Another weird thing is that the National Basketball Association (NBA) has 29 American teams and 1 Canadian one... making it more of an international basketball association. I think another sports league with "national" in its name also crosses national boundaries.
If you take a random person and teleport them between a random mix of Canadian and US cities, I think they'll find it hard to tell the two countries apart. The primary language is English, the accent is the same, the streets and buildings look the same, people watch/listen/read much of the same media, and so on.
One party trick that I practice when traveling in America is to not volunteer information about where I'm from, and see how long I can blend into groups of people and conversations until someone suspects something or asks a direct question. Needless to say, I can last pretty long, and even debated things like US federal politics. The internal diversity of people within the US (e.g. skin color, accent, beliefs) really helps an outsider like me blend in.
Also note that there is a one-way relationship going on. Canadians know more about the US than what's necessary for life. Heck, even the state broadcaster CBC will put out entire news segments (e.g. 5 to 20 minutes) on US-specific issues. Knowing about the US - whether it's major companies, cities, TV series - is unavoidable to Canadians. But ask the average American about anything related to Canada, and you'll likely get a blank stare.
However, some of the differences between Canada and the USA include: Guns(!), personal and state violence, healthcare, social safety net, political polarization, income, prestige, number of big companies, French language, atmospheric climate.