Slate investigates, and the short answer is, because the government stands to lose too much money on lost sales of spirits, which are taxed far higher than beer: the U.S. “takes an excise tax of $2.14 for each 750-milliliter bottle of 80-proof spirits, compared with 21 cents for a bottle of wine (of 14 percent alcohol or less) and 5 cents for a can of beer.”