Bourbon barrels are only used one time due to a combination of tradition, economics, and legal requirements.
Traditionally, bourbon distillers have always used new oak barrels for aging their spirits. This is because the charred interior of the barrel imparts flavor and color to the bourbon, and using a new barrel ensures that the whiskey takes on the maximum amount of flavor and aroma from the wood.
From an economic standpoint, the bourbon industry is highly regulated and competitive. Distilleries must follow strict standards set by the government, and using new barrels for each batch of bourbon ensures that the product meets these standards and maintains consistency in flavor and quality.
Furthermore, the cost of new oak barrels can be quite high, and using them only once helps to reduce costs for the distillers. Once the barrels have been used for bourbon aging, they can be sold to other industries such as the wine, beer, or spirits industry to age their products.
Finally, there is a legal requirement that bourbon must be aged in new, charred oak barrels. Once a barrel has been used to age bourbon, it can no longer be used to age bourbon again and still be called "bourbon" under federal law.
In summary, bourbon barrels are only used one time due to tradition, economics, and legal requirements. While the practice may seem wasteful, the barrels are often repurposed in various ways, making them a sustainable resource.