Another alternative hypothesis for you to consider: there is already so much beauty, novelty, and interesting diversity in the US that it takes a slightly bigger push to get past this local maxima. No offense, but if you chopped out and stitched together all of the interesting bits of Canada you would have a land mass that was about the size of California and with a similar population; in the US we have 49 other states to tool around in as well that present a much greater diversity in terms of environment and inhabitants. We get all that without needing to use a passport, exchange currency, or try to learn to speak like a tourist in another language. It is not the same thing as real international travel, but it is a close-enough approximation for some people that it serves as a reasonable substitute.