The rest of the world doesn’t seem to like it as much as us. We already had a national sport in baseball. But then, some way some how, tackle football became this huge phenomenon. I mean, Super Bowl Sunday is basically a national holiday. This even extended to our colleges so that football became even more important than education. It’s the most popular high school sport throughout the country. How did this develop, and what’s prevented it from gaining this much popularity around the world?
You may be interested in this post by /u/lord_mayor_of_reddit which also links to some older posts, including one of mine on the origins of Gridiron Football in the US.
As a primer suffice it to say that in the 1870's to 90's when the American sporting landscape firmed up much closer to what we would recognize today that there was no gurantee who would be on top. But both baseball and football had several things going for it, including ease of acquiring the needed materials to play, and interest from influential groups(remember the Ivy League schools are who invented modern American football).
Nor should we look at it as an either or thing with Association rules football, rugby, or Gridiron football. All evolved at roughly the same time period from a primordial soup of ball games played on foot across the British Isles with many towns, or schools having their own variations, a tradition which was imported to the US as well. And the codes did borrow from each other, Rugby influenced early Gridiron football notably when Harvard played a pair of games against McGill in Canada in the 1870''s and found some things to like in their early Rugby rules(its how we get from the Scrum to Scrimmage for instance). But there was never any organized effort to export American rules Gridiron football the way Soccer had with FIFA which did much to help it spread, though the existence of other Gridiron codes like Canadian Rules, and similar ones like Aussie Rules, Gaelic, and of course Rugby, show that the style of game certainly had plenty of international appeal, just not as much the specific rule set that developed in the US at the collegiate level in the later half of the 1800's.