eminem's still very hot right now, and the rap genre is huge in the US. more so than rock. While both men are fantastic actors, the current trends lean towards more popularity for Eminem. And the controversy surrounding him makes people wonder what he'll do next. Therefore, the interest lies huge in this movie.
By the way, it wasn't a bad movie at all. Loosely based on his life. Some parts were boring, but it's worth a watch.