Why is a driver who has fallen asleep at the wheel & caused an accident not charged by police?

I was the front seat passenger in a serious multiple car (head on) accident in 1999. The accident caused many minor to moderate injuries to those involved. The driver of the car i was in caused the accident by falling asleep at the wheel! I learned after the accident that the driver had a history of falling asleep at the wheel! Why (in Canada) was the driver not charged by the police?