The idea is that a pitching prospect, no matter how elite, is subject to the risks of injury. Pitching prospects as a whole reach the Majors at a lower percentage than position players. I think one way to illustrate the point is by looking at the pitchers drafted first overall. Bryan Bullington, Matt Anderson, Kris Benson, Paul Wilson, Brien Taylor, Ben McDonald, etc. Andy Benes was the best of the lot.
Another place to look is Baseball America's top 100 prospect lists. They have done the list since 1990. In that time, the top pitching prospects in baseball have included Todd Van Poppel, Brien Taylor (twice), James Baldwin, Paul Wilson, Kris Benson, Rick Ankiel (twice), Jesse Foppert and Edwin Jackson. Hardly a group that has racked up multiple All-Star appearances.
Generally, pitchers are unpredictable in nature, and what looks like a sure hit isn't as sure as the best position prospect.