Because the first half, well, the premise does half of the work if you see what I mean. It may be a brilliant premise, but once you hit on the idea, it's not that hard to flesh it out - not nearly as hard as it is to throw things open in the second half and tackle not only Jack's traumatizing experiences in the world outside, but also his mother's attempts to pick her life back up, his grandparents being torn between love and disgust by his presence, etc.

I would argue that while the first half was what made people sit up and take notice of the book, the second half was what got it on the Booker shortlist and all those glowing reviews.