Because there is no proper evidence to show that it would make any additional contribution to road safety.
When people have drunk so much that they are dangerous on the road ... and they still get in a car and drive ... people like that aren't going to care if the limit is 84, 50, 20 or zero. They will still drive, and they will still cause accidents.
Dangerous drink drivers don't care about the limit ... they gamble on not getting caught.
So, while it would be popular with the vocal public, and maybe a crowd pleaser, lowering the limit here would be a waste of time and money.