Because a movie screen has a width to height ratio of 1.85:1, while a TV screen has a ratio of about 1.78:1.
There are three options: show the black bars, cut off the sides of the film, or stretch the film's picture vertically (but if you do that, the images are stretched).
Many movies are in 2.35:1, leaving very large letterboxing. 1.85:1 will leave small bars. Some older movies from the early widescreen years (50's) are even wider.