Its because his ERA pitching primarily against the very weak hitting ALW (ie the A's and M's) was 4.40 and someone here mentioned that his ERA vs the Yankees is 6. He would be just cannon fodder out there. We don't need more cannon fodder: we already have Miller and now Silva.
Unless, of course, we are clearly in a rebuilding year, Red Sox style, and the owners are already surrendering the season to the better ALE teams.