Sonny Bill had some exposure to Union as a kid. Plus he transitioned via French Rugby which is a Z grade competition.
Burgess didn't have the time to transition correctly. Still, represented his country and played a lot of first grade.
Besides, they are two completely different footballers. SBW sits back and waits for the right time to inject himself into the game. Burgess wants to be involved in everything. This is less than ideal when you're playing a game where 10 touches of the ball and 20+ tackles is considered a high work rate.