Accountability
If matters of National Security have information related to corrupt elected officials, who decides what happens to those officials? Do they get off scott-free because the public can't know the information because it has matters of National Security in it? Are there laws or courts for cases like this? Kind of like a National Security Tribunal?
If the answer to that question is that elected officials get off scott-free because it's matters of National Security, then that means elected officials are above the law. That is not the purpose of the Constitution or National Security. There should always be accountability.