The answer to this can be found in Chapter 18. Specifically, it can be found on pages 356 and 357 in the paperback edition of the book. As is typically the case in this book, the reason for the minimal efforts of the American Indians has to do with geography. Diamond identifies five “major disadvantages” that the Americans suffered from.
First, the plants that Americans grew were simply not as good as those of the Eurasians. Americans relied on corn, which has much less protein than the cereals of Eurasia.
Second, the Americans hand planted individual seeds rather than sowing them by broadcasting (throwing handfuls of seeds). This meant that Americans planted more slowly and could not plant as much.
Third, Americans had to till by hand rather than by plow. This meant that they could not till as much area. It also means that they could not till areas where the soil was too hard to till by hand, even if those areas were fertile.
Fourth, and relatedly, the Americans had no large domesticable mammals. This meant they had nothing to pull plows or provide manure for fertilizer.
Finally, they only had human power, and not animal power, for various farm tasks. This, too, made it harder for them to conduct agriculture on a large scale.