Answer
Due to the increased breaking stress that elephant bones must endure.
Work Step by Step
In the case of human bones and elephant bones, the material is the same, but the shape and size of the bones differ. The elephant has thicker legs than a human, which implies that the bones in the elephant's legs have to bear greater stress than the bones in a human's legs.
To bear the increased stress, the elephant's bones have evolved to be thicker and stronger than human bones.
