Why Transformer Rating is In kVA,why Not in KW?

In Simple words,
There are two type of losses in a transformer;
1. Copper Losses
2. Iron Losses or Core Losses or  Insulation Losses
Copper losses ( I²R)depends on Current which passing 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 k VA,Not in kW.