Actually genetics should be one of the main reasons why football is more popular globally than American football or basketball: Except from the goalkeeper, most roles in a football team don't benefit from an extreme physique (e.g. being too tall has adverse effects in many situations).
Edit: But to answer your question, I think culture stands in the way:
- You can earn much more than your average NBA player but its much harder as an American to get to that level, because you need to get out of the US early on to become a contender (Infrastructure helps)
- somehow football has been marketed as 'unmanly' in the US
- American soon-to-become-athletes have many more options (NBA, NHL, NFL, MLB) whereas in most nations there's at least a clear no. 1 sport.
- Not being American (but wearing my stuck-up hat), but could it be that there's also a race-connection (i.e. caucasians more likely to choose ice-hockey, african-americans more likely to choose American football/basketball, latin-americans more likely to play baseball or football)?