Why transformer rating is measured in KVA or MVA only, justify it?

Operation of a transformer is highly dependend on the temperature at which it operates and the temperature is mainly depended on its losses, which are copper and core loss. since both depends on transformer current and voltage respectively its rating is done mva. 

In transformer there are two types of losses

1) Copper losses: depends upon current (I^2)R 

2) Iron losses/Core losses: depends upon voltage

In order to show these losses transformer is rated in KVA or MVA.