Probably because there aren't any poisonous snakes anywhere else in the world. There is no such thing as a "poisonous" snake, since a poison is a substance that has to be ingested(swallowed)or inhaled in order to be harmful. There are VENOMOUS snakes, but Maine's climate limits snakes of any species. So only a handful of hardy and relatively small species live there. Snake venom most likely evolved as a means of subduing potentially dangerous prey animals, like large rats, and most of Maine's snakes prey on small animals that are relatively defenseless, like frogs or small fish.