People with dark skin who live in the northern hemisphere are not able to synthesise sufficient vitamin D from sunlight. This deficiency prevents the uptake of calcium (among other things) which reduces bone/teeth density and therefore strength.

Fluoride is just one atomic number from calcium and therefore very similar in structure but there is a difference. When calcium joins in the body it creates a lattice structure like a cube, which is uniform and strong but when fluoride is present in the body it takes the place of calcium and the resultant lattice has deformities which show in symptoms such as fluorosis.