The answer to this can be found in Chapter 15. Specifically, it can be found on pages 311 through 313 of the paperback edition. The question you ask is taken verbatim from p. 311 and the answer to the question is found in the subsequent pages.
The main reason why the Australian Aborigines did not develop these things is that they were not able to develop agriculture. Diamond tells us that things like metal tools and complex political societies only emerge in places where agriculture can support sedentary populations. Australia could not.
This brings up the second main reason. Australia had a small population because agriculture could not flourish there. This meant that there were not very many people who could potentially invent technology. What people there were were also spaced out into many groups that did not interact much with one another.
Diamond emphasizes throughout this book that societies need to have good geographic luck so they can develop agriculture. They need agriculture to develop civilizations that have things like metal tools and complex political systems. Australia did not have geography that was conducive to agriculture so its people remained “backwards.”
This brings up the second main reason. Because Australia had no agriculture, it could not have a large human population. This means that there were fewer people who could invent technology.
One of Diamond’s major points in this book is that only societies with good geographic luck get to develop agriculture. It is only those sorts of society that get to have things like metal tools and complex political systems. Australia was not lucky. It did not have the sorts of animals and plants that were needed to create an agricultural economy. For this reason, it did not develop a complex and modern society.