Copyright protection is the only fact-based response to the question. The other comments may be true, but copyright protection is the legal reason why this book is not yet in the "public domain" when it can be offered for free. However, I do think that charging $12.99 is excessive for the only Kindle edition of Gatsby offered.
Under U.S. copyright law, a copyright lasts 70 years after the author passes away. Just guessing but that provision was probably included in the law to provide for his wife and children. Gary P. Landry, author of "River of Mist and Light" and "The Bridge Tender."