Because hard liquor doesn't have carbs. beer even light beer has tons of carbs. And alcohol lowers bs over time, so if your drinking hard liquor it will drop without it raising, but beer will raise it then slowly drop it.
Totally agree with**** but watch out for "flavored" vodka as it usually has a lot of sugar built into the flavor. Straight alcohol without sugar blocks the sugar released from the liver so this can lower your basal insulin needs.