What are the reasons behind the failure to develop a vaccine for HIV?

a virus mutates and so is constantly changing, this makes it very difficult to determine how best to attack or counter the virus, without knowing or predicting the mutation in advance it is almost impossible to effect a cure- science literally needs to replicate the genetic mutation of the virus in order to study it's habits and mutation sequence, only then could they develop an effective cure. part of the problem is economic, much of the HIV virus is concentrated in the poorest countries in the world, the cost of research and development by pharmaceutical companies runs into billions of dollars, a drug company will take an average 0f 8 -12 years (approx) to fully develop a drug- this includes clinical trials on humans, by law a company only has a 20 year copyright or patent on a drug, thereafter competitors can and will manufacture generics of the same drug and sell them at a much cheaper price but without incurring the research and development costs- it stands to reason then that developing a cure for such a complex virus such as HIV will be costly, the cost would be recuperated in the selling price , a price that most developing countries could not afford to pay on a national health scheme.given that the development company only has approx 8 years to recover the cost you can imagine how much a single dose might be- basically the reward incentive from the developing world is not there- they would want a hand out and as CEO of a large drug company you could not oblige , not without upsetting the shareholders and seriously compromising your own job.

The same reason there is no cure for cancer there is a lot of money in it

The virus is "smart" it is constantly changing. It works in many different steps, each step giving it a chance to mutate. By the time, if ever, a vaccine is made you would be dealing with a different virus than what you have a vaccine for. All they can do right now is develop medications that stop each step from occuring. And learn more about the virus...