The AC-130 is a gunship designed to
circle its target. Because it's target is always on its left side, all of its guns are fixed such that they aim to the left of the aircraft. If its guns were mounted side-side symmetric, only half could fire at its target at any one time.
When combat can happen in any direction why make any side weaker.
Because you're making the other side stronger.
All designs have their limitations: weight, cost, size, shape, materials, etc. By making all sides equal, no side is "stronger". Since there's this thing called "rotating" that allows a ship to face its enemy, if a ship evenly armored in every direction combats a ship that's strong in one direction (presumably, the direction facing its opponent), then the evenly armored ship will be at a disadvantage.
Real world examples of symmetric and asymmetric defenses:
Castles are immobile fortifications, so if one had asymmetric defenses, an attacking enemy would flank it and strike its weakest point. Tanks and armored vehicles aren't immobile, which allows them to always face their enemy. That's why they're more heavily armored up front--an example of front-back asymmetry.