Calcification. Your teeth are harder than your bones due to the need for them to resist abrasion, cut and grind food, and last a lifetime since you only get one adult set. Bones are constantly remodeling, and need to be a bit forgiving and flex, up to a point. If the bones didn't flex they would easily break and that would be problematic.
Function. Tooth enamel is the hardest thing in your body because of everything that teeth do. Eating, chewing, tearing food, exposure t o hot and cold, speech, we demand a lot of our teeth.
Enamel vs bone. While tooth enamel and bone have some similarities, they have different compositions and therefore different strengths. Tooth enamel is the hardest and most highly mineralized substance in the body. It is 96% mineral. Bone is not a uniformly solid material, but rather has some spaces between its hard elements.