With the two sports in question, there are many differences with the sports. one, American football is a lot more physical and aggressive which makes you have a higher risk to injury. two, American football has many many more details to the sport with techniques of offenses and defenses, etc. Soccer isn't as fun to watch to many people, plus you can only do so much with a ball kicking it around towards a goal.
Yes, soccer is the most played sport in the world and therefore probably the most popular with the most people playing the game too. however, there is still the fact i stated earlier that anyone who knows anything about american football knows to be true; American football has much more to the game than soccer does. soccer has some fancy tricks and such but there is only so much strategy and technique and all that that can go into such a simple game.