Keep in mind Niagara Falls beats most cities quite handily in international visitors (Toronto, Van and Chi). It just wasn't included in the survey.
One thing to consider is, that Canada and the USA are very large countries. When overseas visitors are planning a tourist trip (as opposed to business or visiting friends/relatives), they usually plan a trip to a country first. If they are planning a trip to the USA, Chicago is simply farther down most people's list of places to visit in the USA than Toronto is on their list of places to visit in Canada. And Toronto is very close to Niagara Falls.