A few processes to explain moral progress (but probably not all of it): a) Acquiring new knowledge (e.g. the knowledge that chimps and humans are, on an evolutionary scale, close relatives), which leads us to throw away moral judgements that make assumptions which are inconsistent with such knowledge. b) Morality is only one of the many ends that we pursue, and as an end it becomes easier to pursue once you are amply fed, watered and clothed. In other words, improvements in material conditions enable improvements in morality. c) Conquest of one culture by another means the morals of the conquerors get transferred to the conquered (to some extent). Similarly, migration and higher levels of general exposure between cultures means practices that are viewed as immoral by much of the rest of the world are under much pressure to be abolished.