Well, the musical is one of the most successful of all time, so I assume that they expect the movie to attract a lot of the same crowd, and if they messed it up then none of those people would come.

I read the book many years ago. It was good, but very gritty, but I didn't understand how they could turn it into a musical. Until I saw the musical and realised that they made a *lot* of changes to the book.