Why is it bad to drink when on antibiotic therapy?

Adverse reactions. In some cases, the combination of an antibiotic with alcohol can have adverse reaction such as nausea and vomiting. In other instances, drinking alcohol can affect the metabolism of the antibiotic.
Liver/kidney. The main reason to not drink when taking them is to not impair your liver and kidney function for proper metabolism of the antibiotics. Also when you drink too much you can compromise immune system and that needs to be working well when on antibiotics. A social drink now and then is fine but hardcore partying should be avoided when taking any meds.