The answer is: It's too complicated for anyone to know with any certainty if crime is actually decreasing, never mind the why. Anybody who can say with certainty about either is lying. Too many factors.

• Increasing surveillance / intelligence based policing ("pre-crime" if you will)
• Changes to methods on reporting on crime statistics. (is a "hate incident" a crime??)
• Changes in numbers of police (no witness no crime)
• Changes in whether the public choose to report a crime. I know I'd never bother, but maybe people my age 30 years ago would.
• Change in decisions of when to investigate crimes (I know for instance >60% of reported crimes here don't get even basic investigation. Do any of those count for a crime rate?)
• Changes to de jure / de facto definitions of crime (eg the many crimes police use their discretion not to pursue).
• Shifting between criminal and civil law. (eg you killed all those people but just pay a fine because it's a civil thing not a crime)
• Changes in propaganda / social consciousness (it's your fault your poor and starving so being a criminal is bad).
• Lead.
• Maybe criminals are just really good at hiding their actions now I'unno.
• And so forth.