should be decriminalised It should not be a crime on it's own, but it should still of course be made clear that driving while drunk is bad and all the usual driving laws apply.
There are many reasons. Mostly just because there's no proper way to enforce drunk driving laws on their own. If the driver of a vehicle is driving illegally, police will pull them over. If not, then there's no way to tell if the driver is drunk without stopping them anyway.