No surprises here: like most good sociology, this study demonstrates what most thinking people had worked out for themselves.
The fall in crime rate is because of the decline in the number of young men as a percentage of society. Simple demographics. Risk takers with little stake in society and too much testosterone get into trouble - or the armed forces.
The "war on drugs" is not mentioned in this article but is also responsible for large numbers of incarcerations for petty offences. Legalising marijuana - just like ending the prohibition on beverage alcohol - would cut the number of 'offenders' at the stroke of a pen.
But prisons are popular. With legislators - because they think that "getting tough on crime" is a vote winner. With rural communities - who need the employment that new prisons bring. With corporations who have muscled into the corrections business and are making out, well, like bandits. With good people, who are increasingly worried by a media that insists on "if it bleeds, it leads", who do not believe that the crime rate is falling and feel threatened especially when they see young men in hoodies hanging around with nothing to do. Young black and latino men especially.
What is also not mentioned is the way the US appeals system works - or rather does not work. Many of those incarcerated have not committed the crimes they were accused of. But because a jury could be persuaded - or because they decided to "cop a plea" to get a lighter sentence - they are behind bars. Once convicted the burden of proof shifts: the convict is assumed to be guilty and must prove he did not do what he stands convicted of. That is a very hard test indeed - and there are men on death row, exonerated by DNA evidence who are still awaiting release. Apparently in the minds of US attorneys there is no such thing as an "unsafe conviction".