Because Turalyon was instrumental to defeating the Burning Legion, which was the original purpose of his character? Killing him not only completes the Windrunner trifecta but gives his son Arator more time to shine. God knows this story could do without more humans.
It's not like he was robbed of any purpose. He defeated the Burning Legion with his allies, then helped the Alliance win the Fourth War. What else do you want him to do, sit and give quests until the end of time?