The answer to this question can be found starting on p. 148 of Guns, Germs, and Steel. There (in Ch. 8), Diamond explains why New Guinea and the Eastern United States were not able to produce as much food as the Fertile Crescent did, even though all of these regions had agriculture. Diamond tries to show that geographical factors (rather than any characteristics of the natives of these regions) caused the differences in food production.
Beginning on p. 148, Diamond lists three problems with New Guinea’s biota. First, he says, it was not home to any “of the world’s 56 largest-seeded grasses.” Because of this, the New Guineans were not able to develop a cereal crop. Second, there were no large animals in New Guinea. Because there were no large animals, New Guineans did not have a good source of protein in their diet. Finally, the root crops that did grow in New Guinea could not grow well at higher elevations, where many New Guineans lived. These three geographical factors meant that farming in New Guinea was not as productive as farming in the Fertile Crescent, which had many cereals and large animals. As Diamond says at the end of p. 149, in New Guinea,
food production was restricted by the local absence of domesticable cereals, pulses, and animals, by the resulting protein deficiency in the highlands, and by limitations of the locally available root crops at high elevations.
Beginning on p. 150, Diamond examines the Eastern United States. He says that it was home to four seed crops and a kind of squash that could be made into containers. This, he says, is quite good. However, there were still major problems. He points out that some of the seed plants had very tiny seeds, which made them hard to cultivate. Another causes hay fever and skin irritation and has a strong odor that many people dislike. Because of these problems, agriculture in the Eastern US did not develop much until around 1100 AD, when beans, corn, and squash made their way up from Mexico, giving the Native Americans of the area a truly productive set of crops that they could grow.
In short, the difference in production between the Fertile Crescent on the one hand, and New Guinea and the Eastern US on the other, was caused by the fact that only the Fertile Crescent had wild plants and animals that could be domesticated and could serve as the basis of strong agricultural economies.