I've read that fans of the book don't really like the movie, while it had an appeal to mainstream movie goers. Maybe that affected the box office.
I don't know much about it but does WWZ actually have enough fans to make a real difference in the box office? Positive or negative.
I don't know much either, but according to
Publishers Weekly, the WWZ book sold over one million copies, as of 2011. His previous book about zombies sold 1.4 million. I guess that's still a lot these days. Good enough to rate a movie, obviously.
The other thing is, I think the movie is actually doing
better than expected.
The Hollywood Reporter says they'll likely be a sequel. Link to story, below.
http://www.hollywoodreporter.com/news/paramount-brad-pitt-set-sights-573814