I wonder how much of it is a player's body, or physical traits that you can't really teach or train. For example, when I was in high school I was a first baseman, but I would pitch occasionally. I'm a really big guy who is relatively tall, but I was born with exceptionally tiny hands for my size. I had a strong enough arm for high school, but it was always really difficult for me to learn secondary pitches because my hands were so small and that made it difficult to manipulate the ball the way I wanted to.
I just wonder how much pitch quality can be affected by inherent traits that you really can't change like hand size or arm length.