You could turn this question on it's head and ask why American Football, Baseball and Basketball aren't so popoular in the rest of the world.
It's all down to what we've grown up with. Until maybe 10-15 years ago very few countries showed sports from abroad on television, but since cable and satellite TV became the norm there's far more air time to fill (and what better to fill it with than two-three hour long sporting events) and so we are introduced to others' cultures.
Video games have also played their part. I didn't have a clue about the rules of American Footy until I played a John Madden game. I knew the basics of basketball before I played NBA on the playstation.
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