[Fomite]

Among other reasons is that HIV is extremely bad at replicating itself without error. Which means, especially combined with the long-term nature of infection, a lot of genetic variation within a single person's viral ecology.

Generally, it's much easier to develop a vaccine for more stable viruses.

We might be able to target some sort of antigen on the virus that's highly conserved even with that mutation rate, but that's a big if.