The answer to this question can be found in the Epilogue of Guns, Germs, and Steel. Specifically, it can be found on pages 410 and 411 of the paperback edition of the book.
In this book, Diamond explains most things through geography. This issue is no different. The Fertile Crescent had developmental advantages because of its geography. Sadly for the people of that area, though, its geography was not “good enough” to allow it to maintain its advantages.
On p. 411, Diamond says that the Fertile Crescent
…had the misfortune to arise in an ecologically fragile environment.
He says that the environment was good for early agriculture, but it was much too fragile to maintain and support a large population. Its people cut down its forests and it lacked the rain to regrow ground cover. This led to erosion, particularly because goats kept the grasses from growing back. This meant that
…valleys silted up, while irrigation agriculture in the low-rainfall environment led to salt accumulation.
In other words, the Fertile Crescent was not fertile enough. Its geography was not good enough to support a large civilization. Therefore, it lost out in terms of wealth and technology to other places, like Europe, whose geography was better.