Might be the same reason heavy plasma torpedos fire from the center of the ship (though I think that's on every ship, not just the D'deridex), whatever that reason may be.
I think that has something to do with CRF, for whatever reason, forcing single cannon weapons [being regular singles and turrets] to fire from aft firing points. Since single cannons have no aft firing point [at least on any ship I've used and observed them on], they use the pivot point, like anything with a missing hardpoint [you'll see it alot using holoemitters on ships with special hardpoints, such as the Andorian ship's Wing Cannons]. Likewise, targetable torpedoes aren't assigned a hardpoint and so fire from the pivot point. Most likely the reason for all this is simple laziness.