High potential pitching prospects can bust and become #5 starters or nothing at all. It's happens a lot. Niese is a sure-thing #2/#3 for your team and has no arm issues whatsoever.
Not all teams are looking for a 38 year old ace for one guaranteed season. Every team in baseball could use a guy like Niese. Niese would be the best pitcher on a team like the Royals.