Why Transformer rating KVA not in KW?

Transformers are rated in kVA or VA and not in kilowatts. Because, It is independent of Power Factor. We can see the reason in this article

TRANSFORMER

Electrical equipment’s like motors, Lamps, Heaters etc., are rated in KW. In that cases, Depends on the type of the load its power factor may vary. Example : if it is inductive it will have the lagging power factor. Current draws by the load may vary depends on the power factor. That is why they are rated in KW.

In case of Transformer, It is an energy transfer device that transfers power from the primary side to the secondary side without altering the energy level. If the Energy loss occurs. It will be Iron loss and copper loss. Iron Loss depends on current and core loss depends on voltage. It is independent of Power Factor. The transformer does not alter the power factor of its output power.

Also, The loads that will be connected to the transformer are unknown to the manufacturer at the time of transformer design. If the transformer is rated in KW, there may be confusion regarding the type of load being connected. In actual practice, the pure inductive (lagging) and pure capacitive (leading) loads do not exist, i.e., every load has some resistance even it is inductive or capacitive. Hence, the practical loads draw both active and reactive powers from the supply.

The temperature rise of a transformer is directly proportional to the apparent power (kVA) which flows through it.

Note : 

1. In actual practice, electrical devices which act as the source (such as alternator, transformer) always rated in kVA to make their operation independent of the load power factor 

2. Electrical devices which act as the load (e.g., motors) always rated in kW and draw the current from the source according to their power factor.