[jballanc]

Two main reasons:

* Genetic drift vs fixation. As bacteria reproduce and die, the relative frequency of genes tends to remain fixed in the absence of selection. In layman's terms: if 0.0000001% of bacteria in a population have antibiotic resistance, they will likely all die before passing it on. If 10% have resistance, there's a much better chance of it sticking around. The more antibiotics prescribed, the more you enrich the population for resistance, the greater chance it sticks.

* Many antibiotic resistance genes put bacteria at a disadvantage relative to the same bacteria without resistance. If antibiotics are not present, then resistance will eventually be selected against and disappear. Overprescription of antibiotics ensures that this doesn't happen.