Because the level of promiscuity among heterosexuals here is not at the level it is in Africa.
There is little homosexuality in Africa, so the virus thrives within the pool of infected heterosexuals. In addition, the rate of STDs of various types is greater in Africa. This provides the open sores for aids to be transmitted between heterosexuals.
I don't think there is as much of a bridge between the drug-using/homosexual population here and the heterosexual community. They cross paths via drug use (injectable) mainly by heterosexual women and their partners.
Africa is only a generation or two from wide-spread polygamy. The notion of monogamous sex simply doesn't exist.
Women (especially wives) have no say in their own sex lives, as far as use of condoms etc. SO if their philandering husband comes home infected, they don't have alot of power to prevent their own infection.
It is very disgusting to read some of the comments here on FR by posters who make assumptions about which they know NOTHING.
Someway Uganda has found something that works. The best thing about abstinence is that it will help to build up stronger marriages as well as prevent the transmission of aids.