It all comes down to welfare.
When chickens are kept in larger numbers the birds inevitably have less space, this boosts production and profit at the expense of welfare and ultimately hygiene.
EU rules prohibit the use of chlorine and set out a minimum amount of space, lighting and ventilation for poultry houses but in the US there are no such laws. As a result chlorine is used to wash bacteria such as Salmonella and Campylobacter (important causes of food poisoning) from the carcasses. Such practices allow producers to continue to rear birds in poor conditions and promote poor hygiene practices.
American poultry may be cheap but at what cost to welfare?