How is the number one reason in this poll: Most American's don't understand the game? It's the simplest fucking game on the planet. Your team tries to kick a little ball into a little net whilst the other team tries to prevent you from doing so. That isn't fucking complicated.
At it's very core, the whole sport is incredibly simplistic. Couple that with how cheap it is to play (dirt + ball + economically disadvantaged youth = game), and that explains its popularity in the rest of the underdeveloped world. America is the richest nation on the planet. We can afford better sports.