Re: Unwritten rule
A number of reasons
1. Their games were built with the senior game in mind-- and what gets rewarded on the WTA is not always what is rewarded on the junior tour.*
2. Great junior results often put a great deal of expectations on young players-- leading to mental fatigue, early burn out and lack of motivation.
3. Players with WTA tour potential stop playing juniors relatively young (age 15 or 16) therefore their games have not yet developed enough to get great results against girls who are as old as 18.
*I actually think that this is why the Williams sisters are doing so well now. While girls their age were flying around the world, neglecting their training, developing poor habits (powderpuff serves), developing mental issues etc. Venus and Serena were simply working on their game in a safe and supportive family environment.