As
@britbox said, his passive play never allowed him to do better. I watched him many times and it always used to piss me off watch him just push the ball over the net. The answer for why so passive, hard to say. He never really found a good balance between passive and agressive play. At times he could strike winners better than anyone and then just retreat to being passive again.
He could have done better in his career if he really went that one step further, changed approach a little bit. Stan did and won slams, but Gael seemed happy on the court doing the show and highlight reel stuff. It's undeniable that competition was brutal, but he could have made at least one GS final. Worse guys than him made it.