Surprised Canadians would pick the US as a tourist destination to begin with. Europe is cheaper and more fun. America's one advantage: nature, is matched and at times exceeded by Canada. Flights to warm places like Miami and SD take just as long as Mexico or the Caribbean. Other than NYC and Utah-area national parks, I can't think of unique reasons for Canadians to vacation in the US specifically.
Looking at the currencies, it might be a bigger difference.
Also looking at flight distance, it might be further.
Visa requirements coudl also vary too.