A big chunk of Canadians don’t want to join a country that is intolerant of non-English speakers.
A big chunk of Canadians prefer the monarchy (which is why they founded the country to begin with – English Canada started with the refugees from the USA in the 18th century).
Candians prefer socialized medicine. They don’t favor a muscular foreign policy.
Why would the US want a country where the “conservatives” are similar to Democrats, and the Liberals are way to the left of Obama.