If we will try hard to improve only the weaker playing skills, it could impact the skills that already were good by making them worse. Therefore, the performance of players will decline. Also, there is a chance that players will not ever be able to achieve a higher level of those weaker skills.
We should focus on improving the strong playing skills, so we will keep the uniqueness of players.
Look at the graphs below:
SK 
EN 
FR 
DE 
ES 
IT 
JA 
KO 
RU 
TR 
ZH 
