The answer to this question can be found in Chapter 6 of Guns, Germs, and Steel. There, Diamond challenges at least two traditional assumptions about the transition from hunter gathering to farming. The first of these assumptions holds that it was natural for people to want to make this transition and the second holds that farming was never actually invented at a given time or place.
The first traditional assumption assumes that all people would want to transition from hunting and gathering to farming if they could. We assume that a farming lifestyle gives people a much better quality of life because we know (or at least we believe) that our lives are better that those of hunter gatherers. On pages 104-5, Diamond challenges this assumption. He says that life was actually harder in early farming communities than in hunter gatherer bands. It was not until farming became more established and more productive that life in farming communities became easier.
The second traditional assumption is that farming was invented at specific time and place. At the very least, we believe, someone discovered how to farm. The people who discovered/invented farming did so all at once and would have then become farmers where they had previously been hunter gatherers. On pages 105-6, Diamond challenges this assumption by arguing that farming was developed in a gradual process. He says that the people who developed farming did not necessarily even make a conscious decision to do so. Instead, Diamond says, farming “evolved as a by-product of decisions made without awareness of their consequences.”
In Chapter 6, then, Diamond says that we are wrong when we assume that farming was invented or discovered and that people would obviously want to become farmers if at all possible. He challenges these assumptions, saying that farming developed gradually and without conscious decisions until it was, in a sense, necessary for people to farm. In the rest of the chapter, Diamond describes how and why this transition happened.