Answer
Drug holidays, from an evolutionary perspective, are not intelligent. Especially in retroviruses like HIV, the replication of active strains restarts very quickly after the drug has been suspended. Even these short periods allow for the higher virus load to have a higher chance of mutations that might lead to new resistancies that then could not be targeted by the medication.
Work Step by Step
Retroviruses are especially evil by pretty easily overpowering our immune system with rapid mutation and multiple surface- forms. The medication normally suppresses the replication rate and keeps the viral load to a minimum. Allowing for the viral replication to restart within the effected cells allows for millions of new options for how the virus could get resistant to your current countermeasure.
