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.
What's more, if you've drunk alcohol and are still perfectly capable of driving, you can still drive drunk and get away with it. And the decision to drive while drunk is made while drunk, so laws aren't doing much good there either, drunk drivers will drive drunk no matter what - however, not all of them will cause a danger.