Because calcium sulfate is an insolable salt so it cannot be prepared by the excess method (adding carbonate to acid) Insoluble salts are made by precipitation reactions

@@DrMonaSeif doesn’t A produce the same salt?

Calcium carbonate reacts with sulphuric acid to produce calcium sulphate but because calcium sulfate is insoluble so it will Precipitate over the crystals of calcium carbonate prevent further reaction that means the reaction will stop so calcium sulfate cannot be prepared by this method