It’s the same reason Yao Ming will never win a NBA championship: The beast is missing. You have to have that lean mean machine make up, to get that killer instinct to win the top prize. That’s partly the reason Sharapova has two GS, despite many weaknesses.
The beast may be obvious like with Serena or dormant like with Federer, but without it the big prize will not oblige.
Why do you think Nadal and Djokovic are thriving and Gasquet is languishing? And why Murray will eventually rise.
Comparison between Shaq and Yao will drive home the point for all you challenged folks.