Two reasons I can think of. One is that it might not have been clear in the film why Dumbledore cast a spell on Harry, or that he was susceptible to being disarmed by Draco because he used his last free moment to protect Harry rather than defend himself. Another reason is that, in the movie, when Snape appears, Harry chooses to trust him (or to trust that Dumbledore trusts him) and expects him to save the Headmaster. This makes Snape's betrayal all the more cutting for Harry, knowing that he could've acted to do something, but instead put his faith in Snape to save the day.