A great, empirically based, comprehensive look at the devolution of criminal justice in the U.S. can be found in "The Collapse of American Criminal Justice" by the late criminal law scholar William Stuntz.
TL;DR: The trend started nearly 200 years ago, though for various reasons it accelerated in the mid 20th century.
Some reviews:
"The book is eminently readable and merits careful attention because it accurately describes the twin problems that pervade American criminal justice today―its overall severity and its disparate treatment of African-Americans. The book contains a wealth of overlooked or forgotten historical data, perceptive commentary on the changes in our administration of criminal justice over the years, and suggestions for improvement… Virtually everything that Professor Stuntz has written is thought-provoking and constructive... Well worth reading." -- Justice John Paul Stevens, New York Review of Books
"This volume is exceptionally rich, insightful, provocative, and well-written. It is bound to have great influence on academic thinking, and perhaps in time on the criminal justice system itself... Stuntz's book will repay much closer scrutiny than I can give it in a review… [An] important book." -- Richard A. Posner, New Republic