Because the movie was shot with an aspect ratio of 2.35:1, and your TV's aspect ratio is 1.78:1. Your TV is not wide enough to show the entire image without letterboxing. HDTV programs are shot at an aspect ratio of 1.78:1. There is a button on your remote marked P.SIZE. You can use this control to fill the screen, but you will be cutting of the ends of the image.