Why is copper (i) sulfate cu2so4^-2? not copper (ii)?

This is because Copper (I) has +1 charge and Copper (II) has +2 charge.

The chemical formula of an ionic compound consists of cation first, followed by anion. Both cations and anions are incolved to make compound to be nutral or charge entity for example Cu2SO4 is cuprous compound contain Cu (I) where two Cu give only one electron as  

Cu is cation and SO4 is anion

SO4 has a valency of -2

So, Cu2 has a valency of +2

But in Cu (II), The single copper gives two electron to SO4 and form CuSO4 compound named as copper sulphate.

These metalls are called transition metals who can change their oxidation states

you need to have a picture of the atom that way we can answer it