You can answer this question by looking at the last page or two of Chapter 8. There, Diamond gives us two reasons that explain why Native Americans did not domesticate apples on their own.
The first reason is that apples are very difficult to domesticate. They require people to do such things as grafting a branch from one apple tree onto another tree. This means that it would be very difficult to discover how to domesticate apples.
The second reason is that Eurasians had much more time than Native Americans to learn how to domesticate apples. It took Eurasians 8,000 years from the time they got agriculture to domesticate apples. Native Americans started producing food late due to geographical factors. If Native Americans had taken 8,000 years to domesticate apples, they would have domesticated them in 5500 AD.
Therefore, Diamond says geographical luck caused Eurasians to start agriculture much earlier than Native Americans, thus giving them the time to learn how to domesticate apples.