Why do bacteriophages only infect bacteria?
Viruses only infect those cells that have a matching protein to the one that is found on the virus' capid.
This protein + protein combination are often referred to a and ship its' docking station.
They have to match or the virus can not enter the host cell and can not infect it.
Bacteriophages (or 'phages) only can infect bacteria for this reason.
Use of 'phages as antibiotics have become a rather interesting idea and maybe they will be the antibiotics of the future.