The mass death of indigenous Americans and aboriginal Australians were NOT carried out by the European settlers/colonists intentionally.
Indigenous Americans and aboriginal Australians were struck by pandemic of germs and viruses brought by European settlers/colonists that they weren’t immune to. Some people claim that Europeans gave ‘flu blanket’ to the natives intentionally to kill them off, which is largely unfounded.
Unlike native Americans and Australians, Africans do have frequent contact with Europeans and Asians throughout history (war, trade, missions etc), hence they are more immune to Eurasian pathogens, and less fatal pandemic broke out in Africa.