Mainly because gay men and IV drug users were disease vectors.
But there were many other vectors besides needles and unprotected gay sex (see article linked), in fact, the index patient simply consumed bush meat that they probably should have left alone.
Yes, the first widely recognized case of HIV in the United States involved a highly promiscuous gay flight attendant, and this likely sped the spread of aids in the first period in the United States up considerably.
But once non-gay people became infected it did not take all that long before they were the bulk of the HIV positive population.
Even so, gay men engaging in unprotected sex and drug addicts re-using needles or swapping needles with others are at extremely high risk for HIV and Hepatitis contraction.
The United States only has a fraction of the world's HIV or AIDS cases, so it seems plausible that worldwide straight people could still represent the majority of cases.
It does seem difficult to get reliable estimates outside the US & Europe, though.