I think the reason is that it's so centered on the death of a small boy. Maybe he saw too much of his own child in it. Just a thought. I don't think I've ever read exactly why he had so much trouble with the story.

For me, I noticed when all those Satan movies came out (The Omen, Rosemary's Baby, and such,) that I realized that I hated movies (and books,) where there was no way that the protagonist was going to win.

I know. End the book at the end. Don't make it sound as if the story continues, but then don't write that continuation.