why is transformer rated in kVA and not in kW?

1.  Why is transformer rated in kVA and not in kW? 

There are two type of losses in a transformer;

1. Copper Losses
2. Iron Losses or Core Losses or Insulation LossesCopper losses ( I2R)depends on Current which passes through transformer winding while Iron Losses or Core Losses or Insulation Losses depends on Voltage.
That’s why the Transformer Rating may be expressed in kVA,Not in kW.