It appears that it is customary in the US to continue to refer to prominent public figures by the title they previously held, e.g. President Obama, Secretary Clinton etc. I understand that this is done as a professional courtesy. However, what is less clear to me is why certain individuals are still referred to by their previous job titles, even when they go on to hold what are arguably more important and powerful jobs. For instance, John Bolton is still referred to as Ambassador Bolton, and John Kelly is still referred to as General Kelly. Yet as National Security Advisor and White House Chief of Staff respectively they hold two of the most important and influential positions in the current administration. Why do people still refer to them by their old job titles?
Ambassadors and Generals are Presidentially appointed, U.S. Senate approved officers of the United States of America of the highest possible rank (short of being a member of the cabinet), with considerable personal authority to act in their own right on behalf of the United States of America.
An ambassador actually sets U.S. foreign policy in the absence of specific direction from the Secretary of State or the President, and a general can issue lawful orders certain to kill thousands of people without prior Presidential approval.
In contrast, a National Security Advisor merely makes suggestions to the President about what U.S. foreign policy choices he should make, or what military policies the President should adopt, and a White House Chief of Staff mostly funnels information from other advisors to the President and helps the President decide what broad policy agendas the President should pursue.
Both the White House Chief of Staff and National Security Advisor are advisors of the President in the White House office appointed unilaterally by the President without U.S. Senate approval who are not officers of the United States of America. They have no meaningful power other than being designated to give the President advice. And, the President is free to give equal weight to the advice and opinions of people with no official position whatsoever on the same issues (e.g. his children, television news commentators, op-ed columnists in a newspaper, lobbyists, and friends and colleagues from before he was elected President).
It terms of precedence and rank for etiquette and protocol purposes like title of address, all employees in the White House office are on a par with an administrative assistant to the President or the President's cook. Of course, the White House Chief of Staff would be senior to the National Security Advisor, who would in turn be senior to the administrative assistant to the President or the President's cook, within the subclass of people who are part of the White House office, all of whom are inferior to all officers of the United States of America.