Multiple causes. It is not clear exactly what the cause is (higher rates of uninsured, poorer/overburdened hospitals in the areas that serve them, indifference or racism by physicians/hospital staff, poorer self care and more reluctance to use the healthcare system, poorer overall health). Its likely that all of those play a role.
Multiple variables. There are a number of possible explanations for this including genetic predisposition, access to health care, community exposure to resistant bacteria, and racial disparity in certain co-morbid conditions such as renal disease, hiv, cancer, alcohol abuse, and diabetes. All of these impact the rate of sepsis in populations.