HIV is a virus whose genes are rapidly mutating, even in a single individual (due partially to its genome being based on RNA, not DNA). Due to random chance, it

HIV is a virus whose genes are rapidly mutating, even in a single individual (due partially to its genome being based on RNA, not DNA). Due to random chance, it's highly likely that an individual will already have a resistant strain of HIV in their body before they underwent this treatment, so after the treatment, only that strain would be left.