Because although people knew of the attraction and repulsion of objects with opposite and like charge before Coulomb, he came up with a formula that allowed you calculate how strong the forces between charges is given the distance between them and their charges. What was before just a statement like "These two things are pulled toward each other," the formula allowed you to say something like "These two the two charges are attracted by a force of 1.37 Newtons when they're positioned 0.45 meters apart.
