I think it's a case of 'first impressions last' for a lot of people, and the first impressions were from the trailer. The voiceover was cheesy and the clips weren't exactly riveting. I think Clive Owen's monotone voice put quite a few people off too. Compare it with Ray Winstone's acting in the new clip released today:
Plus, many people have their own staunch theories on the King Arthur legend, and most of them don't match with this movie's, hence the storyline being torn to shreds. I've lost count of the number of 'Roman Arthur - WHAT ?' and 'Warrior Guinevere - WHAT THE HELL ?' threads on other forums. People have their theories and nothing else could POSSIBLY be correct.
But no-one has seen the movie as a whole yet, so no-one is in a position to judge. I think Spider-Man 2 will do it some serious damage at the US box office, though.