atty in training says:

that’s because a drunk driving killing is not considered murder (which is usually the only killing punishable by death) but most often considered involuntary manslaughter (killing done with gross negligence — negligence meaning that the actor should have been aware of the high probability of death or serious bodily injury but was not actaully aware of that risk.)
http://www.deathpenaltyinfo.org/article.php?did=144&scid=10