If SIV has been circulating in Africa's primates for that long, (and humans have been butchering primates all along), then what was so different about the mid-20th century that allowed it to hop into humans and spread through the population as lethal HIV?Scientists have ideas, but no definitive answer. Some think the "key factor" was the jump in human use of needles. One virologist, as Moseman notes, says it was more likely tied to the rise of cities:
The mystery remains.
It's possible that before that rampant urbanization, human hunters exposed to primate blood contracted the virus. However, without the interconnectedness of modern society, they probably died before they could spread the virus too far.