Seth Masket has a theory:
I think the soccer issue is one instance where conservative leaders are simply trying to draw a line. They feel they've made enough accommodations to other countries and cultures. They simply do not acknowledge soccer as an American pastime and feel they are justified in trying to exclude it from our main culture. We cannot just keep adding things to our nation and assume that the nation will still stand for anything. Lines must be drawn. This far, no farther.
Or maybe they just think it's gay.