In this book, Diamond is arguing that geography is the major factor that determines which societies became rich and powerful and which did not. Therefore, his answer for this is that white men have more "cargo" because of geography. Diamond argues that it is not race or culture that made white people have so much cargo. Instead, it is the fact that Eurasia had many geographical advantages. These advantages allowed farming to arise in Eurasia. Since farming arose first in Eurasia, Eurasians got the "guns, germs, and steel" that allowed them to gain more cargo than the New Guineans had.
In short, then, white people have more cargo because they were lucky in terms of their geography.