Because the three different states are each is more spread out for example water when water is a liquid is not very compact so it is not very dense but if you boil the water it becomes less deses because the atom spread out meaning there is less matter in the same volume. And if you freeze water with no air in it it becomes ice, ice is more compact because to be solid it has to be more compact meaning more atom in the same volume. That is why matter in different states has different densities
