It's druthers, balance and inertia properties, style and fashion, IMO. I own and use as working guns a pair of Berettas, a 20 and a 12, both with 26 1/2" O/U barrels. Love the way they feel, and they are wonderful bird guns. I equally love a Model 12 16 gauge w/ 28" barrels (with receiver, the equivalent to 30" barrels of a double), and a Piotti boxlock 16 w/ 29" barrels that likewise feels perfect. Oh, and I forgot about the Browning A5 w/ 27" tube that feels great and is a wonderful pheasant gun.
My target gun (Sporting) wears 29 1/2" and feels right also.
So what is the answer, short or long? The trend toward longer barrels, especially for target games, has some merit I think in a steadier swinging piece than previously was fashionable. I think also barrel length fashion is driving much longer tubes than are really practical. Much of this variation is like fashion.
