The correct answer is because there are polyvalent metals.
Explanation:
Most transition metals can form cations with various loads. That is why block d of the periodic table is marked as "variable load". For example, iron is polyvalent, which means "many values", i.e. it can form cations with different loads. For polyvalent metals, we must specify the magnitude of the ion charge. In the case of anions most of the time it is not necessary to specify the magnitude of the charge of the anion.
The names of some ionic compounds include the charge of the metal ion while others do not because there are ionic compounds that are able to form more than one type of ion especially for metals and these compounds should be distinguished properly to avoid confusion. For instance, the metal cation of iron, we have Fe2+ and Fe3+. Ionic compounds with metal cations have a charge that is invariant from a compound to another does not include the charge of the cation. However, for ionic compounds of a metal with a charge that would vary in different compounds should include the charge of that particular metal cation.
