instead of "USA"...why not "USOA" ???

In proper grammer, the word "of" isn't capitalized unless it's at the beginning of a title. So it's not often used in abbreviations, probably because the point get across with or without its presence.

Why Not Usa

well it is for the simple reason that "of" is such a small undefined word and does not have no real meaning in USA it is not a very inportant word in this matter

In acronyms such as USA words like "the" "and" "or" "of" are understood.