The reason is to provide the entire picture, as the director intended. Filling the screen by eliminating the "letterbox" bars, would require cutting the far right & left portions of the picture. A while ago, on Turner Classic Movies, they explained why letterbox is preferable to cutting the sides. The show had brief discussions by some famous directors and one referred to Da Vinci's Last Supper. If that picture had the sides cut off, there would have only been 6 disciples visible. While this show was referring to 4:3, it still applies to 16:9. So, it all boils down to trading black bars for full content.It's too bad all newer Blu-ray movies weren't all released in 1.78:1, so that the picture would fill all the screen.