It costs next-to-nothing to release this old material, since the product development's already been done and they already own the rights.
Unlike re-releasing old computer games, for pencil-and-paper RPG's there are no concerns about compatibility with modern hardware and operating systems. (Tangentially, a very successful startup has had success at addressing those concerns with open-source software, allowing publishers to generate revenue from ancient games [1].)
To those who say the material, once released, will be massively pirated: Two big reasons people pirate material are (1) the lack of a legal purchasing avenue, and (2) they can't afford it. This move takes care of (1), and as for (2), the main audience will probably be older players looking for nostalgia, who have a lot more disposable income than pencil-and-paper role-playing's traditional market of middle school, high school and college students.