Macbeth question....?

I took a test today, and there was and essay question I don't think I got right. (In Act 4, Scene 2) I'm pretty sure it's scene 2; it was "Why do you think Shakespeare chose to play out Lady Macduff and Son's murder, as apposed to having it be reported like King Duncan's?" I know in the movie they show King Duncan being murdered, but it is reported (by Macduff, I believe) that he was killed, there was no "Macbeth stabs Duncan" in the text.