Antibacterial medicines (Antibiotics) commonly block biochemical pathways important for bacteria.These inhibit the growth of bacteria or kill them..
Antibiotics do not work on viruses because viruses do not use biochemical path ways such as bacteria. viruses use metabolic pathway of host. 

Behind these questions reasons are as follows:
Viruses enter our cells and use our machinery for their life process.this means that there are a few virus specific targets to aims at.