Rating of Electrical Transformer is in VA (Volt Amperes ) because:

Maximum current carrying capacity of the transformer is determined by the thickness of the conductor and Maximum voltage supplied to the winding depends on the insulation strength of the conductor

Manufacturers of Transformers and alternators does not know at what power factor the consumer uses the machine and the nature of the load (capacitive, resistive and inductive) connected to the machine. Hence they rate the transformer depends on the maximum current carry and the voltage to be applied. This rating of the transformer corresponds to apparent power of the machine (VA). Also iron losses in transformer depends on the the voltage applied and copper losses depends on the current flowing through the winding. These losses are independent of the power factor at which the transformer operates.

On the other hand induction motors are rated with real power (Watts) as the machine operating at defined power factor at full load is pre determined.

Eg: Consider a transformer can carry maximum current of 50A and max voltage applied is 200V. Then

Power rating of the at full load (Unity power factor) = 50x200 = 10kW

Power rating of the at full load (0.5 power factor) = 50x200x0.5 = 5kW (This means load having 0.5 power factor can connect maximum of 5kW to the transformer. 5kW is the full load of the transformer)