Why does London not use the word death?

Without knowing the exact quotation you are referring to, I have no way of knowing exactly why he would not use the word death.  However, the word death is often "euphemized" by saying "passed away" or "gone to a better life" since the single syllable sounds somewhat harsh.  What might be going on is that the dogs die because of the harshness of the conditions; survival of the fittest is certainly true in this environment and Buck, while the fittest of the team, is badly abused by Hal, Charles, and Mercedes until even he is too weak to move until Thornton saves him.