Northern Europeans have the overall best genetics for exceptional limit strength.
Blacks do seem to be more muscular (sinewy and lean), sure, but not capable of building a lot of natural, dense muscle mass. Plus they usually have bird frames. Not conducive to being hugely powerful and strong.
Just look at the geographic locations from which each come, originally.
Heck, just consider the shelter issue: Northern Europeans built houses out of stone and heavy lumber, blacks out of leaves and bamboo sticks.
And food: Whites ate steak, blacks ate bananas and ants.