Why does Magwitch die in Great Expectations?

Even though Abel Magwitch is condemned to death by the court for violating his exile, his illness and injuries ultimately spare him from the hangman's gallows. During his attempted escape down the Thames River, Magwitch gets into a fight with Compeyson (ch. 54). During this struggle, he falls into the cold river and receives two broken ribs and a punctured lung. He never recovers from these injuries.
During the trial (ch. 55) and after the death sentence is handed down, Pip makes daily visits to Magwitch in the infirmary. Magwitch is in a really bad state and getting worse by the day. Finally, right before Magwitch succumbs to his injuries (ch. 56), Pip tells him that his daughter is alive and a beautiful young woman. Hearing this, the old convict dies with a final pleasant thought in his mind.