Via NYT Arts Beat, here's why. Because Salinger said so:
I keep saying this and nobody seems to agree, but “The Catcher in the Rye” is a very novelistic novel. There are readymade “scenes” — only a fool would deny that — but, for me, the weight of the book is in the narrator’s voice, the non-stop peculiarities of it, his personal, extremely discriminating attitude to his reader-listener, his asides about gasoline rainbows in street puddles, his philosophy or way of looking at cowhide suitcases and empty toothpaste cartons - in a word, his thoughts. He can’t legitimately be separated from his own first-person technique. True, if the separation is forcibly made, there is enough material left over for something called an Exciting (or maybe just Interesting) Evening in the Theater. But I find that idea if not odious, at least odious enough to keep me from selling the rights.

Transcript of the letter is here.
I tend to agree. It sort of reminds me of Harper Lee who is very insistent that the film version of To Kill a Mockingbird is the only one that will be. She was so involved in the process of making the film and selection of Gregory Peck as Atticus, and so pleased with the outcome, it would be a real crime if the movie were ever remade!
And Salinger is correct, I believe, in the strength of the written word with this novel.
Let it be.
