Why the rating of transformer is given in kva not in kw?

the consumer power factor is unknown and pf depends on cinnected load so its rating in kva.

because the power factor of the transformer is depending on the load thats why we use kva not kw.

There are two type of losses in a transformer;1. Copper Losses2. Iron Losses or Core Losses or  Insulation LossesCopper 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 kVA,Not in kW.

it is obvious, due to core loses and copper losses

The copper loss and iron loss of the transformer depends on current and voltage but not the load i.e., phase angle between the current and voltage and so the transformer is rated in kVA. 

Transformer is not a load and has no effect on P.F  and it only transfers the constant power from one voltage level to another voltage level without changing the frequency.

Since both the losses viz copper loss(depends on current) and iron loss(depends on voltage) are independent of power factor, that is why a Transformer rating is in KVA, but not in kW.