The answer to this question can be found on pages 16 and 17 of Guns, Germs, and Steel. In essence, what Jared Diamond says is that many of the difficulties encountered by people in areas of the world that have been colonized can be attributed to different rates of development over the long sweep of history.
Diamond begins his answer to this question at the bottom of p. 16. He tells us that
much of Africa is still struggling with its legacies from recent colonialism.
He goes on to say, on p. 17, that in many other places in the world, there are civil wars going on. He says that in those places,
civil unrest or guerrilla warfare pits still-numerous indigenous populations against governments dominated by descendants of invading conquerors.
Diamond goes on to say that many indigenous peoples have had their populations reduced so much that there is no way they can seriously resist the descendants of their colonizers. Even in those places, however, Diamond says that the indigenous people are “increasingly asserting their rights.” Finally, Diamond says that many native languages are dying out as people increasingly speak languages such as English, Russian, Chinese, and some other languages.
All of these facts about the modern world came about because some people colonized other people. Diamond believes that this happened because of the different rates of human development in different parts of the world. He devotes most of Guns, Germs, and Steel to asking why these rates differed from place to place.