This looks like a question more suited for an ask science or ask biology sub. Better luck there.

I can't speak to the natural history of the virus's evolution, but can address the historical factors in its early spread in the US.

Randy Shilts's And the Band Played On is a good overview of the start of the AIDS epidemic in the US. HIV becoming established in the gay male (not LGBTQ) community in the Western world, particularly the US, did have historical as well as biological causes (note that the transmission patterns in Africa are/were quite different, mainly heterosexual, and there are western patterns of drug and heterosexual prostitution-related transmission as well). The sexual revolution and the gay rights movement took homosexuality out of the closet--especially in big cities--and sexual excess became somewhat of a celebration of gay rights. There was no fear of pregnancy to restrain men having sex with other men (and therefore no reason to wear a condom), it was a lot easier to meet other gay men when you're out of the closet (although the closet also plays a factor, as those men often seek out very anonymous and unsafe sex) and STDs, other than herpes, were pretty much all seen as being cured with a short course of antibiotics (which were developed a couple of decades previously). Combine this with easy air travel, and you have a recipe for a sexually transmitted virus to spread pretty easily and widely once it gets into the community. In fact, the famous "patient zero" (although probably not the index case in the gay community, a very early super-spreader) was a very attractive and very promiscuous French Canadian flight attendant, who had sex with a lot of men in a lot of cities.

So: antibiotics and lack of fear of pregnancy, easy cure of STDs create the illusion of safety. The gay rights movements makes it easier for gay men to meet for sex (and also work together as activists and to support each other once the epidemic was established: see, Gay Men's Health Crisis, still operating). Air travel, and particularly an early superspreader who was constantly traveling by air, entrenched the virus in the community well before anyone knew it existed.