One obvious answer is that many more people are in prison than in the past.
Experts differ on the size of the effect, but I think that William Spelman and Steven Levitt have it about right in believing that greater incarceration can explain about one-quarter or more of the crime decline…
Imprisonment’s crime-reduction effect helps to explain why the burglary, car-theft and robbery rates are lower in the U.S. than in England.
The difference results not from the willingness to send convicted offenders to prison, which is about the same in both countries, but in how long America keeps them behind bars. For the same offense, you will spend more time in prison here than in England.
Still, prison can’t be the sole reason for the recent crime drop in this country: Canada has seen roughly the same decline in crime, but its imprisonment rate has been relatively flat for at least two decades.
Another possible reason for reduced crime is that potential victims may have become better at protecting themselves by equipping their homes with burglar alarms, putting extra locks on their cars and moving into safer buildings or even safer neighborhoods.
Policing has become more disciplined over the last two decades; these days, it tends to be driven by the desire to reduce crime, rather than simply to maximize arrests, and that shift has reduced crime rates…
Another shift that has probably helped to bring down crime is the decrease in heavy cocaine use in many states.
Utopia, you are standing in it! 
Jan 03, 2015 @ 13:32:14
James Q Wilson was the originator of the broken windows theory. Unfortunately for him States that adopted his theory had reductions roughly the same as States that didn’t adopt his theory.
We stil really do not know why crime reduced dramatically.
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