Because a producer from a smaller company (Kevin Smith who worked closely with the two) packaged the deal with a ghost writer (William Goldman). The shit was gold way before the pages were wrapped in the agency's jacket and sent to production companies.

Here's part of a ghost writing contract:

^{Ghostwriter acknowledges and agrees that all information related to this project, including without limitation, its content, writings, work product, audio tapes, notes and diagrams, is of great value to Author “Confidential Information”}. ^{Accordingly, Ghostwriter agrees not to divulge to anyone, either during or after the term of this Agreement, any Confidential Information obtained or developed by Ghostwriter while working on the Book and related project. Upon expiration of this Agreement, Ghostwriter agrees to deliver to Author all documents, papers, drawings, tabulations, reports, audiotapes, video tapes and similar documentation and recording devices which are furnished to or produced by Ghostwriter pursuant to this Agreement. Upon the expiration or termination of this Agreement, Ghostwriter agrees to make no further use or utilization of any Confidential Information. Ghostwriter may only disclose Confidential Information to third parties upon the express written consent of Author. The provisions of this section on “Confidential Information” shall survive the termination of this Agreement.} ^ ^{Likewise, Author will accept sole responsibility and liability for the written contents of this project in which he exclusively provides.} ^

By the way, if anyone actually sees this comment, which I doubt many will, I'm not an entertainment lawyer who worked closely with Lawrence Bender anyone from the film, I'm just some guy on reddit.