"King’s horror fiction often focuses on internal threats, those that can’t be seen or predicted. His best novels explore very human fears and anxieties, elements that are difficult to bring to a visual medium. The novel isn’t inherently a superior storytelling medium (we’ve moved past that reductive argument long ago, especially with the second Golden Age of Television), but it does seem more suited to King’s particular take on horror, where the internal, human terrors are privileged over the external ones. His novels are certainly filled with creatures and monsters of all types, but his most poignant works muse on themes that are of this world, hardly supernatural or ghostly."This isn't true only for horror fiction and the novels of Stephen King, by the way. I've found that many mainstream "literary novels" don't translate well into film.
The first one that comes to mind in this latter category is Richard Yates' Revolutionary Road.
Revolutionary Road is an engrossing novel, but a very mediocre film, precisely because so much of the conflict is internal to the main characters. Without the internal insights that the novel provides, the movie is kind of dull, really.