Why? Why did Sun, Jin and Sayid have to die?
Simple. They gave their lives in service of the story. Because their deaths added to the drama. Because a higher power — the-powers-that-be behind LOST — realized that sometimes story threads need to be snipped rather than tied off. Because TPTB have set the endgame in motion, and after six seasons, this all has to mean something. Plus, the characters died heroic, self-sacrificing deaths — the best kind possible, I suppose.