Completely curing HIV is difficult. The virus is able to hide in various places around the body, known as HIV reservoirs.
Anti-Retroviral Therapy (ART) stops viral replication but is not able to eliminate cells that harbour dormant HIV. So people can be treated successfully and become apparently free from the disease, but HIV bounces back if treatment is stopped and is able to keep re-seeding the reservoirs.
The final stage in defeating HIV is therefore to locate and destroy the lurking virus. If we can do that successfully, we may be able to cure HIV infections entirely.