However, soccer is not a popular spectator sport in the USA because, in my opinion, soccer has been "solved." The fact that the woman's world championship soccer game was decided on a penalty kick after a 0-0 tie proved to me that professional soccer teams have learned to be skilled enough, strong enough, and have enough endurance to prevent the other team from winning. They have simply figured everything out, and that's about it. Soccer has simply become to easy for the pros. Thus, I think the solution is simply to make the field smaller and force the game to be more fast paced. Hockey operates on the same principles as soccer, but it doesn't suffer from the same "slowness" stigma.
The lack of popularity among Americans when it comes to international play is simply that you can't really go to see any of the "away games" to root on the USA. It's easy for the British football hooligans to travel from country to country in Europe cheering on their country. In the USA, you're stuck hopping on a flight across the Atlantic if you want to do that.
The US plays 18 months of WC qualifiers against North and Central American teams and the WC itself has been in both the good ol' USA and Mexico.
So so much for that theory.