OTC medications are only approved if the drug and condition are mild enough that a doctor doesn't need to be involved.
And it gets a little bit silly -- 200mg ibuprofen are OTC, 400mg require a prescription.
With antibiotics there's a pretty good reason for them to prescription only, namely that drug resistant strains of illnesses are becoming a serious problem and it's only made worse by the over use of antibiotics. Plus, you likely don't know what the appropriate antibiotics will be for what ever problem you're trying to treat are.
The problem is, it is a rough drug on your kidneys and has potential cardiac side effects.