The viri that already pass human to human are simiar to existing viri and our bodies are better able to cope with it. It is easier to make up vaccines for this too.

Flu viri mutate easily and readily. It is likely that the thing will eventually jump to humans, most likely through a third animal. And when it does we will have zero immunity to it. So if it comes it is likely to be very dangerous, while existing human flu viri will mutate but are not likely to instantly become so different from existing lines as to be nearly as lethal.

Researching HIV is not mutually exclusive of researching the flu. HIV is not new, and so it does not garner a lot of face time on TV or the front page. It also is not likely to wipe out a continent worth of people inside of a month once set loose. This new flu thing is news because it is something changing in our lives and our environment. I am not putting one as more important than the other, but the flu can kill large numbers in a short time and can spread in a day or two to every corner of the earth.

Another factor is political. There is a segment of society that still looks upon HIV as somehow dirty or sinful. They think that all HIV victims are promiscuous drug users. They fight efforts to send substantial aid. There is no social stigma attached to the flu. Furthermore, flu can come and take down my family, while the likelihood of my wife and I contracting HIV is vanishingly small. The flu thus threatens my family while HIV does not. Not directly anyway.