Bacteriophage are not stable compounds such as chemical molecules (antibiotics, etc...). They evolve and change, adapting to bacterial evolution (or just spontaneously because we're talking about organisms, who sometimes just changes). It may be an advantage from a efficacity point of view, buts it's a big no-no from a regulation point of view.

This makes is harder to have real trials (even if things progress slowly).

Then you have all the biological questions about injecting live virus in an patient and the risk of immune response.

Then you have the complexity of both producing phages in a stable manner (remember: these fuckers have a tendcy to "evolve" on their own) as well as shipping them (require refrigeration all the way, contrary to antibiotic pills).

Source: bacteriophage were the subject of my master thesis, even if it was a while ago.