Re: Pitchers' Development Question
Some pitchers have only one or two quality pitches and are then considered relievers. Other pitchers may be incapable of being effective for multiple innings. Some players are able to get a little more velocity if they are reserved for the bullpen, and the difference in velocity makes the difference between them being a mediocre starter and an effective reliever. That's just the tip of the iceberg on how to answer this question.
There have been many players who are tagged as relievers, but they play as starters in the minors to give them more experience. A few recent cases of this in the Reds organization are Ty Pelland, Ramon Ramirez, and Sean Watson.