Soccer is boring. It is like hockey without the fist fights - zero point. Not enough scoring, not enough meaningful action for a casual fan.
In football and basketball - America's two top sports - there is constant action - basketball has tons of scoring and football has hitting on every play.
Soccer is a good sport for kids - non-contact and tons of cardio, but as an adult sports fan I am not interested. Personally, I would love to see soccer banned from TV. It is like watching paint dry. Hockey, to me is fairly boring too, but at least hockey has checking and the occasional glove dropping action. I would rather watch lacrosse, women's volleyball (more for the scenery) heck I would rather watch women's college softball than soccer.
You may disagree with my sentiments, and I don't begrudge anyone of their opinion, but that is how many Americans view soccer.