joinyouinthesun has just uploaded some behind the scenes photos from The Empire Strikes Back. I am not sure how widely these photos have been distributed in the past (if, indeed, they have at all), but now I think of it I don't remember seeing many on-set shots when the original Star Wars trilogy was released. It's only more recently that images have done the rounds on the interwebs. I am sure that Lucasfilm (and now Disney) carefully control which images are made public.
With the rise of the DVD and the internet there has never been such need for 'extra' content. The lure of exclusive material drives sales and attracts subscribers (which may in turn generate advertising revenue). Studios and publishers working in all media are now very aware that content is king and has immense value. They archive the Making-Of process knowing that every snippet has the potential to garner an audience. However, all this must be balanced against the potential damage that can be wrought by spoiling the magic, releasing designs which could be used in sequels and - worst of all - off-message content.
Star Wars, is, of course, a special case. It occupies a space in modern culture unlike anything else. Perhaps it is so fondly remembered and enough time has passed that photos of Chewbacca grabbing Leia's boobs are not a threat any more.