The thinking behind it, I believe, goes something like this: the production company wants the movie to appeal to all kids, not just the "brainy" ones who enjoy reading. Basically the movie is marketed to the average child while the book is marketed to the academic top 50%. It's complete bunk, of course, since most of those "brainy" kids will come out of the film thinking it was awful because it ruined the book, then go tell their like-minded friends not to see it.