I'd guess most people that are serious about American Studies find it easier to take the course at an American University, and prefer the immersion along with having professors more likely to have experienced American culture first-hand.
On top of that, a lot of people probably aren't interested in American Studies because British people are already exposed to American culture all the time in the form of movies and books. And you probably find it hard to avoid us online because you speak English... which means you may very well be familiar with American culture to the point of knowing what the Fifth Amendment is, whether you wanted to be or not.