The Death Star was a trap[1]. It wasn't semi-complete. It was fully operational[2] and designed to lure in the Rebel fleet to a single location. Destroying the rebels in detail had proved to be too time consuming.<p>[1] Ackbar, Return of the Jedi "It's a trap!"<p>[2] The Emperor, Return of the Jedi "As you can see, my young apprentice, your friends have failed. Now witness the firepower of this fully ARMED and OPERATIONAL battle station!"