You can find the answer to this by reading in Chapter 19. There, Diamond says that we can look at the patterns of language in Africa to know how peoples spread out across the subSaharan part of that continent.
Diamond points to the fact that Khoisan and Nilo-Saharan languages are spoken in very scattered pockets. He also points out that Pygmies lack their own language family. From this, he concludes that speakers of Bantu spread out across Africa, overwhelming the speakers of the other languages. This is why Bantu languages are spoken across a large and unbroken range while the other languages are not.