When transformer rating is in kVA, why load is mentioned in WATTS?
"Watts" of a load signifies the Real/Active power. It is the useful power consumed by it. Reactive power is not useful. Consumers are normally charged by the utilities only for the real power. 

$$P_{real} = P_{app} cos \phi $$
\$ \cos \phi \$ is the power factor of the load, 

While KVA of transformer signifies its rated apparent power. It defines the maximum current you can draw from the primary/secondary for given voltage. Above which, copper losses cause windings to heat up.