Not sure how much this has been discussed thus far, but the psychology of fandom is important here. If you're not raised with a sport, you're less likely to like it. Furthermore, if the culture you live in doesn't care about a sport, you're even less likely to like it. Add in the fact that American Football is so strongly associated with America and suddenly you've got a sport that isn't so appealing to a Frenchman, or a Russian, or a Japanese dude.