SARS & Ebola kills people fast. So, all the government has to do is quarantine the infected and they'll be dead in days. Problem over. HIV kills super slowly.

SARS & Ebola kills people fast. So, all the government has to do is quarantine the infected and they'll be dead in days. Problem over. HIV kills super slowly. Technically it doesn't kill. As a result, it's successfully infected hundreds of millions. If we agree that one chief goal of living things is propagation, HIV is very successful while the fast killing viruses aren't.