Why has no vaccine for HIV been discovered yet?

Work is ongoing. The virus mutates easily, so any vaccine needs to target parts of the virus that cannot change, and this has been a challenge. I like the answer provided by anthony s. Fauci, M.D. Director, national institute of allergy and infectious diseases national institutes of health on may 12, 2012: http://www.Niaid.Nih.Gov/news/newsreleases/2012/pages/hvad2012.Aspx.
Difficult. Hiv virus is a difficult organism- has several different proteins based on which vaccines need to be created. Hiv virus has a tendency to mutate making it difficult for vaccines to work. However monoclonal antibodies show some promise. Still research ongoing - hopefully we will have a breakthrough.