The answer to this can be found in Chapter 15. Specifically, it can be found on pages 313 through 317 of the paperback edition. The question you ask is taken verbatim from p. 313 and the answer to the question is found in the subsequent pages.
One reason why more advanced technology did not reach Australia was distance. Indonesia is rather far from Australia so any contact between the two would have involved voyages across open water. New Guinea is not so far away, but it is still far. Between New Guinea and Australia, there are a number of islands. Diamond says that trade hopped from island to island until it finally reached Australia. That means that Australians were not really exposed to New Guinean culture and technology directly.
Diamond also says that the nature of the environment made it harder for Indonesian and New Guinean technology to be adopted by the Australians. The Australian environment (at least in northern Australia) is not suited for farming. Therefore, Indonesian and New Guinean technology that was suitable for farming people was not suitable for nomadic hunter-gatherers.
Thus, distance and geographic differences were the main reasons why advanced technology did not reach Australia. However, Diamond’s answer is quite long and there are more details than can be covered here. Please refer to the pages mentioned above to find more details.