Because the books are out of print, and could be hard to find/expensive? Just a quick look on Amazon.com shows the lowest price for the 2nd edition of the core rulebook is $57 (American). Why spend money on a product that really isn't that different from the books you already own? (Pathfinder/D&D)
Its unlikely the movie itself will cause people to become roleplayers, and any roleplayers that may want to play in Conans world probably already have a favorite rules system that they will convert rather then learning a new system.