I think the intended purpose of having elections for these positions is to give the legal system more legitimacy at the local level; after all, some federally appointed prosecutor isn't going to be very sensitive to the concerns of the locals in the area where he's appointed, which would affect their willingness to cooperate with the judiciary and the police. The real problem is that there are multiple local groups with sharply diverging preferences, which means the prosecutor ultimately has to choose sides (and in most cases, they will side with the majority).