Why isn't acyclovir used to treat viral infections other than herpes simplex? Why isn't it effective against the common cold or other viruses?
Because it is Herpes-specific...
Herpes Simplex Viruses (HSV-1 or HSV-2) is a member of the Herpesviridae, a large family of related (dsDNA-)viruses.
Upon infection, HSV DNA will be incorporated into the host's genome and can stay there in a dormant state for a long time.
Indeed, once you are infected by HSV (type 1 or 2) it will stay with you for life. The dormant state is known as the "Lysogenic pathway"
When it gets triggered into action (into the Lytic pathway), VAST amounts of copies of the viral DNA are needed to make new virus particles. In order to ensure this happening the viral genome contains a gene that codes for its own version of an enzyme called DNA Polymerase.
All living organisms produce DNA Polymerases, but they are not all the same: the enzyme is more or less Species-specific.
The different Polymerases are divided into 7 Families (categories):
There is more than one Human DNA Polymerase present in human cells: these fall mainly in families A, B an X.
Acyclovir competes with, and inhibits, the HSV Polymerase (Family RT) , but it doesn't affect the Polymerases of the ( in this case human) host cell, as these fall in different families and therefore have a slightly different 3D-structure, and different mode of action.
By the way, Acyclovir isn't only effective against Herpes Simplex : it is also used in the prevention of CytoMegalovirus (Glandular Fever, Pneumonia) and Epstein-Barr Virus, and against Herpes Zoster (Shingles) and Varicella Zoster (Chickenpox).
All these viruses are members of the Herpesviridae, and thereby share the same version of DNA Polymerase ....