L. Frank Baum wrote 14 books about the land of Oz, but the movie was just a dream. Was MGM convinced that adults would consider movie more believable that way? Apparently the thought of a sequel never crossed their minds. Movies (and TV shows) often alter the details of the original source, whether it's a novel or a true story. The reasons vary, but often it just comes down to making things more exciting for the viewer. For example, Sarah Paulson showed off some serious acting chops in the O.J. Simpson miniseries last year.
And so did Marcia Clark in the courtroom. She acted professionally, while Paulson acted emotionally, because that's what the script demanded for each of them. I had wondered how I ever forgot that topless picture detail, but now that's all cleared up. Read two dozen ways that movies (and TV shows) were different from the original source at Cracked.