[WhiteSage]

Vaccines do well for diseases for which your natural immunity can protect you from them after you've created antibodies. HIV works the other way: the virus lets you create antibodies and then it wins the fight by hiding in your immune system and slowing turning the tables.

The virus also stay dormant within your cells, and it is not absolutely clear where, yet. Other viruses which do this (chickenpox, herpes, papillomavirus, ...) are not cleared from the body by the immune system either, but at least the immune system can control them as long as it is healthy enough.

This facts, of course, are no reason to be pessimistic about a cure/vaccine, just explains why it is taking that long. A cure might also pad the way to cure other diseases such as chickenpox and herpes, which may lead to complications later in life.