TWR - "Don't shout at your children, don't correct your children, don't smack your children, otherwise you will get done! who's caused all this?"
There is no one isolated factor that has caused the downward spiral in child rearing.
Some factors that re brought to bear are things like the increasing isolation of your parents, with no role models to guide them.
Generations ago, young couples would have had parents, grandparents, brothers and sisters, all with children, all living in the same street, or maybe one or two streets away, making social contact frequent. Guiding by example, and talking about raising children, young parents got a good grounding in parenthood, and children had plenty of role models outside their immediate family. Modern society isolates people from such influences.
As each family grows, the law of diminishing returns kicks in - bad parenting leads to children growing up with no notion of fitting into society, of kindness and respect, and so the spiral goes down.
Children look for the comfort of bonding and security they don't get at home, and find it in gangs, with their own hierarchies and acceptance cultures - such as carrying a knife, and being prepared to use it.