[kwhitefoot]

Most large transformers are actually unique but this has little to do with the characteristics of the transformer. It is mostly to do with optimizing the design to minimize the total lifetime cost. This requires a new design for each transformer because the relative prices of core steel and copper or aluminium varies over time. When copper is cheap the total cost will be lower if you use more copper and less core steel for instance while the opposite will be true if the copper price is high.

If cost is not as important as prompt delivery the manufacturer can often just take an existing design and repeat it.