In Guns, Germs, and Steel, Jared Diamond identifies parts of the world that are not suitable for food production but he does not spend much time talking about them. This is because Diamond is not very interested in places that are simply never going to be suitable for farming. We can see this, for example, at the beginning of Chapter 5. There, Diamond tells us that there are “large areas of the globe” where “ecological reasons” make food production “difficult or impossible” even today. These are places like the Arctic or deserts which simply cannot support farming. So these are the areas that are completely unsuitable for food production.
But this is not really important to Diamond. What is important to him is why some areas that were potentially suitable for did not actually develop food production very quickly if at all. This is important because Diamond says that places that got food production earliest were the places that became rich and powerful.
Diamond argues that the places that were most suitable for food production in ancient times were the places that were lucky in terms of their native plants and animals. Diamond says that food production developed in some places because there were many native plants and animals that could be domesticated. The places that were not suitable for food production (even if their climates were fine) were those places where there were few domesticable plant and animal species.
We can say that there are two ways to answer this question. First, we can say that deserts and arctic areas were not suitable for food production in ancient times and are often still unsuitable (with the exception of very dry places that can sustain food production if irrigation is used). Second, and more importantly, we can say that Diamond says that areas that did not have many domesticable plant and animal species were not suitable for food production in ancient times. This is why they did not develop food production very early and why they subsequently did not become rich and powerful.