Canada is a far easier country to travel to than the United States. Canada allows far more people to visit without a visa. Please note that U.S. citizens will now require a passport in order to enter Canada.
Citizens of nations such as Greece, South Korea, Israel, Botswana, and Hong Kong would all need to apply for visas to enter the United States...but can freely travel to Canada without any formalities. Mexico and the Czech Republic require visas.
For those who do require a visa, it is still typically much easier to acquire a Canadian visa - for example, while the United States rejects 40 to 50% of student visa applications, Canada rejects only about 10%. ("Second time lucky", The Telegraph of Calcutta, India.) In part because of the travel difficulties there has recently been a steady procession of American organizations holding conferences in Canada. For instance, the American Psychological Association, American Library Association, Association of Trial Lawyers of America, American Philatelic Society, FBI National Academy Associates have all recently elected to hold conventions in Toronto.