The answer to this question goes to the whole point of Diamond's book. Diamond argues that Eurasians invented these things because of Eurasia's geographical advantages.
Diamond says that Eurasia had more plants and animals that could be domesticated than other regions of the world. He says that Eurasia's long east-west axis allowed for the diffusion of agriculture. Because agriculture arose and diffused rapidly in Eurasia, more civilizations arose in that landmass. Since Eurasia had more civilizations that arose earlier, it had more people who could invent things. There was also more time for Eurasians to invent things because they discovered agriculture earlier than anyone else.
Thus, Diamond argues that geographical advantages led to Eurasia's ability to invent so many kinds of technology.