Because it's a behavior that spreads it. Stop the behavior, and the disease will eradicate itself, but our lost society won't do that. The homosexual lobby has a lot of financial power.
As long as it stays within the homosexual community, it's not a big issue like cancer. If people knew a certain behavior was responsible for all cancers - most people would stop doing it.
The plague is homosexuality (sodomy). HIV/AIDS is just one of the consequences.