The Chimbu and the Daribi are discussed in Chapter 13, p. 252 in the paperback edition of the book. There, we can see that Diamond does not know what accounts for the difference between these two tribes.
Diamond says that the Chimbu were "especially aggressive" in using the technology they saw Westerners using while the Daribi were very reluctant to change. But Diamond does not try to explain what it was about either of these societies that made them the way they were. Instead, he says (on p. 253) that societies vary in essentially random ways when it comes to culture.
Diamond is not really concerned with why the two tribes are different. As long as these differences are distributed randomly around the globe (rather than having people from one continent accept change while those from another continent reject it), the differences in culture do not affect Diamond's argument.