For the same reason our understanding of human psychology changes every few decades. Our scientific observation and understanding evolves.
Just as you wouldn't commit electroshock therapy anymore for people with mental disorders, you wouldn't use shock collars today.
Or the same way we don't recommend corporal punishment for children, it's not recommended to manhandle dogs either.
Dogs respond very strongly to both negative and positive reinforcement. Negative reinforcement unfortunately has the effect of making the dog associate stress or physical abuse with conditioned events. Much in the way that a child will stop doing things out of fear, but it builds up mental scarring.
Particularly many people try and condition dogs to not bark or growl by negative reinforcement. This causes the dog to be fearful of expressing itself and go more directly to biting when stressed.
Anyway Tl;Dr science doesn't sit still.
