Because some movies are really terrible. That's not bad in and of it self, but when the movie claims to be something that it's not, a thinking persons movie, or a "historical" thriller, and turns out to be an overly long pile of doo-doo it's going to get creamed.
Now myself I am a fan of the Horror genre, and if you are a fan of horror movies your going to watch a lot of crap. But that's OK, because generally horror movies aren't distributed under the pretense that they are anything other than low-budget, popcorn muching, lurid, cheesy, tougue in cheek fun. In short prentious movies with big name actors, that cost hundred of millions of dollars to make, better be damn good paticularly if they are going to go of their way to offend a large segment of the public.
i shouldn't admit to it, but I actually did like some elements of "stigmata," a movie rife with heresies and Catholic bad-guy plot points, but which made no claims to being true, and was not presented as a thinking-man's movie. (The gospel that stigmata claimed the Catholic Church was violently trying to keep hidden was, in fact, published by Catholic historians. It was the howlingly silly "Gospel of Thomas.")