Probably because it is from a child's book. A librarian I work with just described it a kind of prototype Young Adult Literature book when the genre was just getting off. I just happened to read the book several weeks ago before I knew that a movie was coming out. I thought it was a dreadful book and certainly wouldn't want a child of mine reading it. The author is quite talented and I know good quality writing when I see it. But in the book, she mocks Christianity and praises paganism (although she is a former missionary and wife of a Presbyterian minister) and really makes the rural Southern family look bad.
That I find easier to believe than a prior poster's description of the author as a "Christian counselor." Maybe in this instance "Christian" is merely shorthand for somebody who thinks Jesus was a good guy, which seems to be the lowest common denominator definition used by people such as Hillary Clinton.
I thought the movie was terrific until it felt the need to comment on Christian theology. That seemed to be the tipping point in how I viewed the film as a whole. Suddenly it brought into focus the portrayal of the church-going family as dour and unsupportive, whereas the non-believing family is bright, open and fun. The death of the little girl even plays like a Christ metaphor, because her death seems to bring light and reconciliation to that repressed Christian family.