I read about the therapeutic uses of bacteriophages in Pelczar's Microbiology. It was written that it is reasonable to think that bacteriophages could be used to kill pathogenic bacteria. However, the book also clearly stated that after a lot of research it was found that bacteriophages can't be used to kill bacteria in the human body because they can't survive inside the human body. But no reason was given why.
This question cannot be answered, because the assumption behind your question is wrong. I haven't found a source to claim that bacteriophages cannot survive in the human body. Bacteriophages can and are, in fact, used as a treatment against human bacterial infections, it's called "phage therapy" (1,2,3).
This is, nevertheless, a rather uncommon procedure for a number of reasons:
- Availability: Compared to antibiotics, which you can easily get from any doctor/pharmacy, phage therapy requires special facilities, because very specific phages need to be produced for each disease.
- Safety and regulation: Virus handling requires special safety measures and using them in humans requires a lot of paperwork, safety tests and clinical trials.
- Money: Due to the reasons above, phage therapy is a lot more expensive than antibiotics.
- Lack of research: Thanks to the availability of antibiotics there was no urgent need to develop the application of phage therapy. We know comparably little about how the immune system might react to bacteriophages in the blood or bacterial toxins released by lysed bacteria. We haven't found a way to e.g. quickly engineer bacteriophages for diseases.
With a growing concern about multi-drug resistant bacteria, developing new ways to combat diseases has become more important. Maybe phage therapy will be one way to deal with this more regularly in the future.