When making an abbreviation from the initial letters of something, it is customary to not include the initials of the minor words, like of, the, and and so on. For example NASA stands for National Aeronautics and Space Administration. The “and” is left out. NAACP stands for National Association for the Advancement of Colored People, leaving out for, the and of.
POTUS stands for President of the United States. If that abbreviation were to follow the rules of leaving out the minor words, it would be PUS.
And that might not be too flattering.