Transformers are rated in kVA because the losses occurring in the transformers are independent of the power factor.
The loads that will be connected to the transformer are never known to the manufacturer at the time of transformer design.
Hence it cannot be rated in kW.
The temperature rise of a transformer is directly proportional to the apparent power (kVA) which flows through it.
This is also a principle factor behind that the rating of the transformer is given in kVA and not in kW.