One reason is that a lot of players have incorrect or poor technique when it comes to throwing. This limits their ability to progress. They don't ever learn how to correctly throw basic shots like picada or rebote or even kill shots. Players who do excel tend to have more textbook form and delivery and as a result seem to get better and more effective over time. An extreme example would be Scotty. Scotty could have had all the athletic ability in the world - but with his improper delivery on not only his serve but also both his left and right side throws, he was never going to get out of the early games.
