Do you not know the history of HIV/AIDS? Yes, anyone can carry it, but as far as has been discovered, the first person to get HIV was a gay man. And since gay men have sex with other gay men (unless you're experimenting or in the closet with a female) that's who primarily contracted the disease. One gay man passed it to another gay man and it became a chain of gay men passing it to gay men. Not because gay men are the only ones who can get it but because the first victim was apparently a gay man and that's the community where it became concentrated.
In the 80s when literally millions and millions of gay men were dying of it, people were convinced it was a gay disease. It look a long time for people to realize that heterosexuals could become infected as well.
It's an issue that can affect everyone, but it still continues to be a big deal in the LGBT community.