These are the three reasons of why HIV virus is successful in humans.
1. In most viruses, the genetic transfer of information mimics a host's DNA is transcripted to RNA which is translated into a protein. But HIV is a retrovirus, that carries its genes around in the form of a single strand of RNA, rather than DNA. HIV hacks the cells of the body with help of reverse transcriptase, which allows it to create a DNA version of the viral genome after it invades the cell. The retrovirus genome mutates quickly, its hard to figure out what anchor a drug can latch onto.
2. HIV is a member of Lenti virus genus, the other retrovirus can only infect the cells in the midst of division but lentivirus can infect anytime.
3. HIV virus is hard to destroy because it tends to infect the cells designed to destroy it, lymphocytes are there to fight pathogens, buy can also lie inactive. The lymphocytes can maintain a hidden reservoir of HIV that can bring the infection back to life even after treatment and no drug can touch these reservoirs.