Why are group 5a element have different oxidation state from -3 to +5

Because the oxidation state is an artificial number  

Explanation:

The oxidation state is the charge that an atom would have if all its shared electrons belonged to the more electronegative atom, that is, if the bonds were completely ionic. This seldom happens.

For example, consider NH₃ and HNO₃.

Both compounds are covalent.

However, the N in NH₃ has an oxidation number of -3, because N is more electronegative than H.

Thus, N "gains" the three H electrons and gets a theoretical charge of -3.

The N in HNO₃ has an oxidation number of +5, because O is more electronegative than N.

Thus, N "loses" its five electrons and gets a theoretical charge of +5.

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).