American Football combines a high level of scoring, violence, and marketable personalities. It is a true team sport, requiring a high level of execution by all eleven players on the field on every single play. Unlike soccer (or Futbol, as it is known outside of America), football has a pre-determined time length for its games- something critical in the American commercial environment.
Football fans, like their counterparts overseas, often become fans of a specific team at an early age and this fandom makes them feel a part of the team. But unlike soccer fans, many football fans have never played the game beyond a recreational level. As a result, the professional players become larger than life figures. They are performing on a stage that the average fan could never approach.
Scoring plays a key role in the success of football (and lack of success of soccer) in America. It's stereotypical to say American fans have short attention spans but it's also generally true. Your average American fan wants to see points on the scoreboard and wants the potential for lightening-quick changes to that score. A game that ends 3-0 would be considered dreadfully dull by most American fans and luckily very few end that way.
Hitting is also an essential element in Football's popularity. For years the NFL marketed "Crunch Classic" tapes of the game's biggest collisions. Highlight shows continue to show these hits on a weekly basis. Heavy padding makes hard tackles something that players generally walk away from without injury but is still visually impressive. Concussion concerns have begun to change the style of hitting but hard tackles are still a major draw for the game.
Finally, the NFL has a diverse group of marketable personalities. America is a melting pot of different ethnicities and the NFL offers star players for all of them. No matter what your racial background you can find an NFL star, past or present, that represents that group with pride. Many of those stars also have strong local connections, living in the communities long after their careers are over.