A number of factors:
Poor Marketing: Most people who saw the trailer had no idea what the movie was about.
Poor Reviews: Movies like Bad Boys II and Hancock are pretty much review proof but if your serious drama that's vying for an Oscar gets 27% on Rotten Tomatoes its not a good sign.
Subject Matter: People probably aren't used to Will playing this sort of character or the somber tone in the movie
Previous movies: Those who were disappointed in I Am Legend and Hancock may have thought twice about rushing out to see the latest Will Smith movie.
Nothing other than Will to grab you: Not a well known director or producer and no other big name cast members to draw people in.