When power transfer from primary winding to secondary winding of transformer there are two losses in transformer one is called core losses this is due to the alternating flux in core and second is copper losses this is due to current flowing in the winding when the transformer is loaded.
Output power of transformer is given by P= VICosØ, for different types of load i.e (resistive, capacitive, inductive) cosØ changes so, for same voltage and current output power will different, so transformer is designed to operate at particular voltage and current levels and it not designed to deliver particular output power that is why rating of transformer is in KVA.
OR
As copper loss of a transformer depends on current and iron loss on voltage, Hence total transformer loss depends on volt-ampere and not on phase angle between voltage and current ie. It is independent of load power factor. That is why rating of transformer is in KVA.