The answer to this question can be found in Chapter 15. It is seen most clearly on pages 305 and 306 of the paperback edition of the book. On those pages, Diamond tells us that there were two main problems that faced farmers in New Guinea.
First, there was the problem of the terrain in New Guinea. Diamond says that much of New Guinea is not suitable for intensive agriculture. Much of the island is at very high elevation where it is too high for farming. In much of the low areas of the island, it is too swampy and so people cannot farm. Those people who live in dry areas of the lowlands can only do slash-and-burn agriculture. This meant that only the highlands of New Guinea, between 4,000 and 9,000 feet of elevation, are good for intensive agriculture. Even in those areas, there are only a few broad valleys that are suitable. Thus, one problem with agriculture in New Guinea is that there is too little land where it is feasible.
Second, there were not enough sources of protein among the indigenous foods available to New Guineans. The staple crops, we are told, were root crops like taro that do not have much protein in them. In addition, the only animals that could be domesticated were pigs and chickens. Diamond says on p. 305 that the “production” of these animals “was too low to contribute much to people’s protein budgets.” The pigs and chickens were also not capable of providing highlanders with any way to do things like pulling plows.
So, the two major problems were farming in New Guinea were that A) there was not enough land where intensive farming was possible and B) the indigenous food sources did not provide enough protein to support large populations.