Correctional policies have reflected changes in our social, economic and political environments because our social and economic environments strongly affect our political attitudes and our political attitudes are reflected in the policies enacted by our elected representatives.
As an example of this, we can think about how our prisons have come to be overwhelmed with people in prison for drug crimes. Until the crack epidemic of the mid-1980s to early 1990s, our prisons were less full and our penalties for drug crimes less draconian. With the rise of crack came a demand for a crackdown. In other words, our social environment (one in which there was perceived to be a rise in crime due to drugs) affected our political environment (in which there came to be strong demands for harsher sentencing of drug offenders). This affected our correctional policies to the point where we lock up huge numbers of low-level drug offenders for long periods today.
Our economic situation matters too. When the economy was stronger (and when there were fewer prisoners) we were more willing to pay for programs in prisons. Today, with the weaker economy, we are reluctant to pay for any extras for prisoners. In fact, our economic problems are even starting to push politicians towards reducing the number of people we are incarcerating because we can no longer afford to spend huge amounts putting people in jail if they are not true threats to society.