From my college years I remember that vaccination was hard because the outer shell of the virus is capable of mutating so rapidly that it's hard to make antibodies specific to it.
The difference, as I (also not an expert) understand it is that HIV is unique in that it's so variable that the host never develops immunity to the disease. In most viruses, the immune system eventually develops an immunity and eliminates the disease. Influenza is an example of this; your immune system learns to fight off the strain that you get. The new vaccine every year is for the new strains.
The HIV vaccine mutates quickly enough that the host never develops immunity themselves, which makes triggering immunity with a vaccine very difficult.
