Why is the University of Colorado known as CU and not UC or U of C?
The same applies at Kansas-KU, Missouri-MU, Nebraska-NU, Oklahoma-OU and Denver-DU. “Midwestern casualness,” says former CU historian Fred Casotti. It has always been this way at Colorado, for whatever reason, and at the other five listed above-but seemingly nowhere else in the USA. In the 1950s, there was a concerted effort to eliminate the use of “CU” on the Boulder campus, both as a symbol and in speech, but Casotti said that no one would buy into it. “Nobody would change,” he said. “It’s easier to say than U of C, UC sounds like slang or something (as in ‘you see’), and it was traditional. By trying to eliminate it, they reinforced it.”
(from the official athletics website of the University of Colorado)
Similarly, here we say “UA.” “AU” must just be slang.