Personally, I don't know that 3 att/2 def is better than the reverse, but it is certainly easier, and I think that's part of why the Advanced has a bad rep. Its a defensive ship, and defensive ships are generally harder to fly successfully than offensive ships. Look at the Falcon. Very offensive and so easy to win with some think its broken.
The other reason is that it just doesn't fit in the Imperial line up that well. It would be a great Rebel ship if it wasn't competing for spots with cheap TIE fighters.
I know the math says its overcosted, I'm just not convinced.