Why cant you rate transformer in kW?

Because in case of T.F/Gen we dont know the the power factor and nature of load to be connected to these power sources. And also in transformer the copper losses depends upon current passing through the winding and iron or core losses on voltage. So total losses in T.F depend upon on VA and it is independent of power factor. Thats why they are rated in KVA.

Transformer has two types of losses and these are the core loss and the coper loss. core loss depends on voltage and coper loss depends on line current. The product of current and voltage gives KVA rating not KW. Thats why transformer cant be rated in KW.

Because Transformer have no rotating parts like Generator. In generator/Motor,we consider power-factor,because the air gap between stator & rotor decides power-factor or slip or leakage flux. In case of Motor/Generator P=root3 X Volt X Current X Power-factor = KW. But in case of Transformer P=root3 X Volt X Current X 1 ( Power-factor) = KVA

That is why in case of Transformer PF =1 & Generator/Motor 0.8-0.95 & allways less than 1.

Since the power factor is dependant on the load type, the transformers are rated in kVA and not in KW.

For example let us have a transformer of 11kV/220V rated as 22kVA. The rated current is 100A. Let a load connected to 220V side with a PF=0 draws rated 100A from the transformer the output real power(kW) P(kW)=220x100x0/1000=0 and the apparent power S(kVA)=220x100=22kVA.

Now let the load on 220V side draws rated 100A at a PF=1; The real pwer in this case is P=220x100x1=22kW and apparent power= 220x100=22kVA

As clear in both case the same transformer for same current have the same kVA but different kW. therefore the transformers are rated in kVA

Rating of Transformer is in KVA always and not in kw because it is  Apparent power and not a real power.

Transformer power 

S=p+q

KVA = KW+ kVAR