The best example of a region of the world that is suitable for agriculture today, and which did not have agriculture until recent times, is Australia. Australia today has very productive agricultural lands. Even so, the Australian Aborigines did not farm. Surely, that is a sign that they were backwards and that, contrary to what Diamond says, there are racial or cultural reasons why some people develop agriculture and others do not.
But Diamond refutes this argument. He says that areas like Australia are only good for agriculture now. They were not good for agriculture before the coming of the Europeans. In the case of Australia, the area was not good for agriculture mainly because there were no crops to grow or animals to domesticate. Diamond points out that none of the things that are grown in Australia now are native to that continent. Instead, all sorts of things had to be brought from elsewhere. He mentions wheat, barley, oats, apples, grapes, potatoes, and cotton. He also mentions sheep. All of these were brought to Australia by the Europeans. Therefore, it becomes clear that the Aborigines could not have started to engage in food production because the plants and animals available to them were not domesticable.
As Diamond says, on page 320 of the paperback edition of the book,
…the development of food production in Australia had to await the arrival of non-native crops and animals domesticated in climatically similar parts of the world…