Buehrle was able to get stretched out in the minors where as Prior did not. Prior only pitched 50 innings in his first year in the minors because of a non-pitching related injury. He then goes up to the bigs and 116 and 211 innings in his next to seasons.
More importantly, though, is that Prior has a far more stressful delivery and possibly a weaker arm (again, this has nothing to do with the mechanics). For one, prior had far FAR better stuff than Buehrle. Now, I'm not saying that it isn't impressive how durable he is, I'm simply saying it's not a good comparison. If Buehrle had a similar delivery then I'd say it's a good point, but he doesn't.
The point is that most pitchers have to get stretched out for a couple years until they can go 200 innings every season and some simply don't have the durability to last 200 innings period. Some pitchers have the durability the day the step into pro ball (lincecum, etc), but most don't. It's not the norm, especially for someone with a violent delivery. If they had just limited Prior's innings and slowly built up arm strength they could have possibly saved his career, but the point is that we just don't know. The human body alone is a gigantic and complicated factor that is different with every person.