Question

why carbonate and sulphate convert into oxides

Answer 1

As it is easier to extract metals from their oxides rather than carbonates or sulphides. Thus calcenation or roasting is done.

Answer 2

Carbonates and Sulphates are converted into oxides as obtaining a metal from its oxide is much easier than doing that from a metal sulphide or a carbonate.And also during metallurgy, metal sulphides and carbonates are first converted to metal oxides during metallurgy.