Why did the yankees trade sergio mitre?

Something's confusing me. He had a fantastic record, a marvelous, just marvelous era, and an absolutely fabulous number of strikeouts and a very low/scarce WHIP, homeruns and RBIs. Although he had a very slightly rough post season last year, he was still a absolutely spectacular pitcher to have in the bullpen. Then we trade him for a guy who opted out of his contract with the yankees and joined their rival. What a bad trade, cashman is an idiot.