Same reason you mess with anything, to make the team better. The bullpen is extremely deep now, and even deeper when Suter goes back there. Then there's still Miley, Ramirez, and others waiting in the wings. There's lots of options there to cover Hader's innings with similar results. There are not a lot of options to start and give you 5 good innings. Hader's job as a reliever is to throw gas and strike people out. As a starter, he would need to modify his game a bit. We don't know if he could be an effective or even dominant starter, and how long it would take. The way I look at it, sooner we find out, the better. Look at Hader’s stats in the minors and you’ll see he was a dominant starter who frequently went deep into games before he reached Colorado Springs which is a pitcher’s nightmare (whose idea was that to make them our AAA?)). His changeup seems like a decently serviceable third pitch and he just needs to get more comfortable using it. The way I see it is, would you rather have Hader pitch 3-4 innings every 5 games as he is now or would you rather have him pitch ~6 innings every 5 games as a starter? Starters are more valuable than relievers and they could much more easily spend money on relievers to replace him than they could spending money on starters. There was probably many people who thought Chris Sale should remain a reliever, but then look what happened once they realized he should be a starter. Burnes and Peralta should be looked at as starters too. Yup, just look last year there were only 3 relievers who had more than 3 WAR (Fangraphs). There were 7 starters who recorded over 3 WAR last year that had a 4 ERA or higher. He doesn't even need to be "that" good as a starter to be worth more WAR than the most elite relief pitchers.