That is because Congress decreed in 1964 that “bourbon whiskey is a distinctive product of the United States.” Three elements make bourbon unique: American corn, pure limestone water and new, charred oak barrels.

The Times has a piece about Kentucky Bourbon.

I took a tour of the Wild Turkey distillery a few months ago and I asked the tourguide about this distinctive drink. He was clueless, but said it was the law. You can make bourbon outside of Kentucky, but there are certain limitations on the percentage of ingredients and the purity for it to be labelled as bourbon.