So, first off, I'm not a scientist, or even a very science-y person, and I study the historical, social, and political side of the AIDS crisis. But I have a few things to say about this. There were several factors at play for why AIDS has become such a deadly epidemic. It's kind of a 'perfect storm' of factors that cascaded into each other.
- Incubation period: HIV has a really long incubation period. It can vary person to person, but it's pretty much always a matter of years. People can be HIV+ and asymptomatic - they don't look or feel sick, but can still transmit the virus. HIV had already entered the 'collective gay male bloodstream,' as it were, by the late 1970's (the first reported cases by the CDC happen in the summer of 1981.) So by the time people start getting sick, a lot of people have already been infected.
- Gay male sexual culture: Gay men created a really vibrant culture in urban centers in the post Stonewall (1969) years. This culture was largely centered around sex and (homo)sexuality. Bars, bathhouses, as well as codified forms of public sex (cruising, cruise-y gyms and movie houses, "tearoom trade" (sex in public bathrooms)) were all very prevalent. Uninhibited sexuality was directly linked to gay liberation. Condom use was extremely rare; sexually transmitted diseases were considered 'no big deal.' This certainly made it easier for HIV to spread through the gay male population. Additionally, because of the strong link between gay liberation and gay sex, gay men were hesitant to change their sexual behavior and culture in the very early years of the crisis. There was a pretty profound lack of solid information, and many gay men were unwilling to give up their sexual identity based on uncertain information about how AIDS was transmitted and what really caused it. So you have a virus with a long incubation period, being transmitted within an insular community that values sex and sexuality.
- Government Neglect: It cannot be overstated just how badly the government failed to respond to the AIDS crisis. Because of the conservatism of the Reagan years, and because AIDS initially affected mostly gay men and drug users, the government was extremely slow to respond to the crisis. President Reagan did not mention AIDS in public until 4-5 years into the epidemic. This meant that there was very little research, very little support services, very little information about what AIDS was, how it was transmitted, and how to protect yourself. Homophobia, fear, prudishness, sex-negativity - these all played a role.
All these factors played into each other and contributed to the massive reach of the AIDS epidemic. HIV is already a tricky bastard of a virus. Then it hit a community that was insular, highly sexually active, and loathed by the social and political mainstream. Because of its association with the gay community, AIDS acquired a truly massive amount of stigma. No one wanted to study the gay disease, or be known for studying it. Even removing the 'gay' piece of it, most people are reluctant to talk about sex and sexually transmitted diseases. This combination of factors allowed AIDS to become the massive epidemic it is today.