There are probably many reasons why this movie failed, or close to it, at the box office. Some of them are obvious. The economy is poor and consumers have less money to spend on entertainment. When they do go to the movies, they're much more likely to throw down some cold cash for a franchise that has proven itself to be competent, if not groundbreaking. Del Toro, much as I love him, just isn't a wow-type director that will put asses in seats with his name alone (is there such a director anymore?). Maybe the movie just wasn't as good as I thought it was (I recognize when I enjoy something because of nostalgia and personal taste rather than inherent quality).
But mostly, I think the promo for the movie was just inadequate. I mean, just look at this trailer:
I liked it, but then I liked almost everything about the movie. And as I said in my first post about the subject, I would go see just about anything by del Toro.