When a film is based on a book, why does that not appear in the writer credits (when not the writer's book)?

The 1962 movie "The Manchurian Candidate" was based on a 1959 book of the same name by author Richard Condon. But this does not appear in the screenwriter's credits. When I tried to correct that, I was told that was not possible. Why wouldn't an original source be credited? In other entries there is a nod to the source material.