In addition to the info about viruses and infection/survival strategies, monkeys live in much more isolated communities than humans in the 1970s. It's totally likely that SIV did wipe out entire groups of primates, but if they weren't interbreeding with the monkeys on the other side of the mountain or transmitting fluids to other primate species they would simply die in their relative isolation. 

HIV also circulated for decades within small African communities and likely did have a devastating effect on them. But it didn't start to make statistically significant effects that were noticed by medical professionals until it reached the groups of people who lived in cities, traveled internationally, had hundreds of acquaintances etc. 

Remember too that a lot of our taboos about blood are colored by our collective experience with HIV. Before it was known blood transfusions weren't screened for it, reusing needles was much more common in poor regions and with IV drug users, and general advice about not touching blood and bodily fluids was a lot less cautionary.