This is an interesting question. The most important reason to me is that American are the masters of advertisings. Thus, if you support a sport in EEUU, it will be known worldwide. This is if we take into account commercials!! In Spain, we are soccer, tennis, basketball and F1 fans, because of we have professional players in these sports, on the contrary in the USA, they are masters. For example, If they didn´t make commercials I wouldn´t know American Football.