while soccer is fairly popular in the US, it will always have trouble becoming more popular, and it will never be the most popular here, mostly because of what others have said, we prefer action sports. It is tough for soccer to compete with Am. football (college and pro), basketball (college and pro), nascar, hockey, baseball (although that can be just as slow as soccer sometimes). And for those that didn't hear, MLS attendance was the 8th highest in the world among top soccer leagues last year.
a lot of people are turned off by soccer because of the flopping and acting. I'm always yelling at the TV for players to get the fuck up.
http://www.youtube.com/watch?v=gHebuF8tNEM