The answer to this can be found in Chapter 6 of Collapse. The answer is that Iceland’s problems were caused by all of the factors in Diamond’s framework except for hostile outsiders from other societies. The Viking settlement in Greenland had troubles with all five aspects of the framework, but there were no hostile outsiders in Iceland. Diamond tells us that, among the societies of Iceland, Greenland, and some others that he examines in the book,
…only in Viking Greenland did hostile outsiders (the Inuit) intervene critically.
Diamond says that all of the other factors caused problems for people in Iceland. The Icelanders themselves caused environmental damage, largely by grazing excessive numbers of sheep. The Icelanders’ response to the problem made things worse. Diamond says that the Icelanders were “conservative and rigid” and did not want to accept innovations that might have had many benefits for them. Icelanders were originally able to grow barley, but climate change brought a cooler climate to the island and made that impossible. Trade with friendly neighbors actually helped Iceland and allowed it to escape complete collapse. Thus, all of the factors other than unfriendly outsiders were important to the fate of Iceland.