The transfer of HIV from one host to the next is very inefficient because getting started is hard, but almost none of the immune defenses, even if activated before infection can be sure of defense. Antibody is very strain specific - new HIV variants arise all the time so that antibody to one strain does little to stop a different strain. The Tc response is much of the time advantageous to the virus and can actually enhance the spread of virus in the new host. Singly they are of no use, only as a population both of different specificities of the same type of function, and of different functions. However, the immune cells are not capable of all things, they have to protect most of the time against most pathogens so pathogens that are too effective, as HIV might be when it passes from mother to child and kills the child, results in changes in either the HIV, or in the host.
Somebody has to change eventually.