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.
I have a Kindle and bought the book for 9.00 with free shipping. I will pass it along an many people will read it. It might end up at a community book sale where others will read it. Can't do that with a Kindle and that is one reason I think the Kinle price should be much less. Also that free 2 day shipping is going to cost Amazon a few bucks. Money that Amazon would like to have saved by selling the Kindle version cheaper, but they are not allowed to.
You have the wrong author. Fitzgerald wrote The Great Gatsby. No matter, though, who ever wrote it, I still don't get why it keeps making the lists. I've read it multiple times, and it does nothing for me, as a story, as literature, as prose. Completely overrated, in my opinion.