Why does carbonate

have a -2 charge and not -1?

Why does phosphate have a
-3 charge and not -2 or -1?

Why does nitrate have a
-1 charge and not -2 or -3?

My answer: Based on the Lewis structure, these ions need a certain number of electrons in order to be stable...the octet rule allows the atoms to achieve a noble gas configuration. Is this explanation conceptually correct?

Yes, I think it is but I think the question is nonsense.