Because his story has been told. It's terrible that Two-Face had to die in a Joker story, but bringing him back to life undermines the core of the story. The Dark Knight is about the Joker winning. Every single event that was orchestrated by the Joker, to the very last step, (except, perhaps, the boat's not detonating, but he seems to have accepted that as a possibility) went according to his plan. It was about him twisting the two beacons of hope in the city. To twist Dent into a monster, and to force Batman to kill. Would Batman have won if he had picked differently when he had the opportunity to choose Rachel or Harvey? If I remember correctly, Batman actually chose Rachel to save, but Joker lied to him about which one was where. It seems to me, Rachel is the "Bruce Wayne" choice and Harvey is the "Batman" choice. Anyway, it's integral to the movie that Batman has to kill a friend, and the pillar of Wayne's dream, while he's compelled to not kill the psychopath who orchestrated the whole thing. If Dent shows back up, that negates the center.
Besides, Batman would have checked to see if Harvey was dead.