Not (Originally) for Public Sale
As an industry, advertising plays a key role in the movie-making business. There are literally hundreds of movies released each year, and the studios are vying for public interest in their particular projects. To assist the theaters in promoting the various releases, the movie studios produce and provide materials that are designed by their advertising departments to assist theaters in putting on a successful movie campaign. These materials are not released to the public. As such, they are printed in limited numbers --- only enough to accommodate the local theaters. They are strictly advertising materials – meant to be discarded after the movie's run.
Of all the reasons why people collect movie art, none seems to compare to the basic instinct in all of us that we would like to have something that most other people will never have. Because of the limited numbers produced; because of the number of them destroyed, either intentionally or through age; because they are not meant for public sale; and because they can only be acquired though limited avenues, movie art offers rarity in a world of mass production.
NOTE: For clarification purposes, we are addressing legitimate theater advertising materials and not commercially produced posters and the like.