Because it is only feasible to advertise a few names in association with the movie, and most of the billing already goes to the actors.

Just from reading a script there are a million possible interpretations. In fact the script will most likely be tweaked at the directors discretion as they go along.

Think of it this way, the classical play version and the modern day set Romeo and Juliet are essentially following the same script. They are, however, vastly different movies.