Answer: This is to prevent resistance to any single drug.
Explanation: The retrovirus is a very smart virus that is hard to manage. It uses the enzyme Reverse transcriptase to convert its RNa to DNa. HAART means highly active antiretroviral therapy which includes a combination of several drugs whose job is to stop the conversion of the viruses RNA to DNA at different points. It will be easier for the virus to outsmart one drug with one mechanism of action but it will be more difficult to outsmart different drugs working independently of each other. This will reduce the chances of resistance to the medication.