Because some folks in publishing have a Hollywood mentality. (Remember Alexandra Ripley's egregious "Scarlett"? 105 one-star reviews out of a total of 385 at Amazon.)
Link to June 3 cnn.com post, "Lawsuit targets 'rip-off' of 'Catcher in the Rye'".
Good advice from Salinger: "There's no more to Holden Caulfield. Read the book again. It's all there," the court filing quotes Salinger as saying in 1980. "Holden Caulfield is only a frozen moment in time."
I've been meaning to take Salinger up on the re-reading offer. But now I'll have to wait until after I've completed my extended visit to Rabbit Angstrom's Brewer, Pennsylvania.