Why is it difficult to develop antiviral medications?​

Antiviral drugs are a class of medication used for treating viral infections.[1] Most antivirals target specific viruses, while a broad-spectrum antiviral is effective against a wide range of viruses.[2] Unlike most antibiotics, antiviral drugs do not destroy their target pathogen; instead they inhibit its development.

Antiviral drugs are one class of antimicrobials, a larger group which also includes antibiotic (also termed antibacterial), antifungal and antiparasitic drugs,[3] or antiviral drugs based on monoclonal antibodies.[4] Most antivirals are considered relatively harmless to the host, and therefore can be used to treat infections. They should be distinguished from viricides, which are not medication but deactivate or destroy virus particles, either inside or outside the body. Natural viricides are produced by some plants such as eucalyptus and Australian tea trees.[5]

your answer is "the 1950's"