According to Guns, Germs, and Steel, how does a civilization adopt or decide to use a writing system?
You can answer this question by reading in Chapter 12. There, Diamond says that there are two ways in which civilizations adopt an writing system (rather than creating one of their own).
Diamond says that writing has spread in two main ways. The first of these is what he calls "blueprint copying." In this method, one culture borrows a writing system directly from another. The one culture essentially just takes the other culture's writing system and uses it for their own language. The second is called "idea diffusion." In this method, a culture hears about the basic idea of writing and then develops its own completely new system of writing. Diamond says that societies adopted writing in one of these two ways.