One theory is that it is related to the type of onion bacterial pathogens that are common in the different growing regions across the United States.
In Georgia, the most common bacterial pathogen appears to be Pantoeaananatis, which occurs along the leaves of an onion plant where foliar spray of non-systemic bactericides may directly come in contact with the bacteria and kill it.
In the other onion-growing regions of the United States, the most common bacterial pathogens (e.g. Pantoeaagglomerans, Burkholderia spp., Enterobactor spp. etc.) occur deep in the leaf axils in the neck of the onion plant where they are protected from foliar sprays of non-systemic bactericides.
For more information, on commonly found types of bacteria among US onion-growing regions, click here.
For more information on how onions become infected with bacteria, click here.