Damn... last time I answered this question so many people hated me. Sorry if I'm insulting you, Americans. I am NOT applying this to everyone. If you're going to waste time writing a million responses to me, I'm sorry this affected you so much.
I think the reason is because Americans are used to watching sports like American Football, Baseball, Hockey, etc. that does not require as much of an attention span.
Therefor, when they see Soccer it may be boring to them and they can't keep up because they are used to not have expand their attention span as much.