Because the criminal justice system relies on punishment as a mechanism to stop and deal with criminality, it is economically, pyschologically, socially, and philosophically destructive.
For one it is extremely hypocritical to punish people for anti-social behaviors by putting them in an institution that tends to be far worse than the crime being perpetuated. Robbing a bank can lead you spend a decade worth of shower rape, bullying, isolation, psychopathic donut security, brutality, poor standards, torture, and depression. How is that justice? Prisons do not stop crime statistically, in fact 90% of people who come out of prison turn out to be more violent and 50% will go back to prison. Incarceration also leads to higher unemployment, harms all people affected by the incarceration, and is destructive to the people in and out of prison. Not even rapists are given justice because they go through far worse than whatever they did to their victim, which is both cruel and injustice even if the rapist deserves castration.
It is just hypocrisy to say "don't do bad things or we do bad things to you as well," or "don't kill or be killed," or "drugs ruin your life so we ought to ruin your life by incarcerating you when you do them."
There are numerous studies in the realm of psychology that demonstrate that punishing people for bad behaviors is counterproductive and destructive.
We would be doing every person in society a favor, and would harm no one, by destroying all prisons and replacing them with mental hospitals and locked up, safe facilities that would provide therapy and medical treatment to all bad people until they're no longer a threat to themselves or others, sometimes that would be for life but it's for more equitable, non-hypocritical, and productive than our current failure of a criminal justice system.
We need to rehabilitate all criminals. Not punish them.