Why is calcium oxide and not sulphuric acid use das a drying agent?

That depends on what it's being used for. If it's a dessicant (especially in a dessicator), then sulfuric acid is one of the best. But if you need to dry a liquid by direct contact, then mixing it with sulfuric acid (another liquid) is not a good idea b/c it will contaminate and probably even react with whatever you're trying to dry. Calcium oxide is a solid with low solubility in non-acidic and organic liquids, so it will not contaminate or react.