The answer to this can be found in Chapter 18. Specifically, it can be found on pages 354 through 356 in the paperback edition of the book. The answer to this is very much like many other things in the book because it has to do with food production.
One of Diamond’s major themes in this book is the idea that food production makes societies powerful. Diamond argues that societies that start producing food first get more time to build up and become strong. Therefore, they tend to become the conquerors. This is what happened in the case of Eurasia and the Americas. Eurasia got food production long before the Americas did.
Diamond says this very clearly on page 354. There, he says
The most glaring difference between American and Eurasian food production involved big domestic mammal species.
What Diamond is saying here is that Americans had no big domestic mammals and therefore could not really produce food as efficiently as the Eurasians could. He says that Eurasia had 13 species that could be domesticated. By contrast, the Americas only had one, and that mammal, the llama/alpaca, only lived in a very small area of the land mass. This is important because it means that natives of the Americas did not have meat and milk to give them protein. They did not have anything to pull plows or provide manure for fertilizer. Because of these things, the Americans could not create an agricultural economy as soon as the Eurasians could. To Diamond, this is the most glaring difference between the two land masses.