I think it has to do with the nature of the antibiotic. Some antibiotics can really irratate and damage tissue when injected. I presume that when a choice is available (i.e. injection v. IV) many choose injection because of lower costs. Then again I'm just speculating. I've never done injections but doing IV has been a real nightmare with regards to the insurance. I'm a bit curious about bicillin shots....as my IV has to be pulled in the next 2-3 weeks. Hopefully some others will have some good insight.