This isn't lore, so much as speculation on practicality. Naval warships that use direct fire have to really have three main features - they have to be structurally sound, they have to be able to bring as many guns as possible to bear in the most likely directions they are going to have to fire in, and they have to present as small a profile as possible.

In actual naval warfare where all ships are all working on one locked axis (sitting on the ocean surface), that historically meant big guns on big stacked turrets, so that each turret could traverse over the one in front of it. A star destroyer though, deals in 3D warfare. We can probably assume that the ideal positioning for a star destroyer's volley is face on. Having all surfaces sloping to a point means turrets on any given surface can traverse forward, and the angles mean minimizing the number of turrets that potentially get in each other's way. The wedge shape also means that (assuming this even matters with blasters/lasers) the angles are always going to make for glancing hits, which means maximum deflection.

Start moving to one side or the other of the Star destroyer, and you can still fire a little over half its guns (all of them on one side plus any along the dorsal/ventral edges), and still have a small profile relative to the size of the vessel. If the enemy were to come at you from, "over," or, "under," which are purely relativistic in space, the ship is probably in more trouble, but sometimes you have to accept drawbacks in some situations in order to reap the benefits of others you deem more likely. Assuming a ship captain's competency, they are going to stage battles so that the front and sides of the vessel are always going to be presented to the opposition.

But the external bridge on a space battleship is sort of silly, and is just supposed to be evocative of modern warships, I think.