Why did we take rating capacity of a transformer in kV a?

Because we don't know the power factor of load attached to the output of transformer so we use KVA rating instead of KW 

KVA shows the total power or juice the transformer can provide to the load. Now this powercan be used as real power KWA or as reactive power Kvars it upto the load. In short the load will define the fate of the available power.

Actually the (Kva) is a unit for measuring  the apparent power and the apparent power in (kva) dividing in two parts. the first part is the active power in (kw) and the second part is the reactive power in (kvar). the electrical network healthy measured by  the load consuming for active power or reactive power. So in the perfect situations the loads should be100% of the active power and0% of the reactive power. but the perfect situation not available in the truth because of different types of loads and the efficiency of the loads. So we should take into our consideration the consuming for the reactive power in (kvar) in our designs and electrical equipment like transformers, generators, switchgears .....etc.

1-Because the Load of transformer is not constant.

2- The losses of Transformer depends on the current ( Iron losses ) and voltage ( Magnetic losses )

3- The transformer doesn't has power factor

I agree with the answers shared by talented professionals, I think I don't have anything new to add.

Thanks

Cu(Copper) loss of transformer depends on current and Iron loss on voltage.Hence, total transformer loss depends on volt-ampere(VA) and not on phase angle between voltage and current i.e. it is independent of load power factor.

That is why rating of transformer is in kVA and not in kW.

power factor may vary from place to place, from service providers also, thats why,