what's up with the late 30s?
1. Rapidly-improving social conditions.
2. CCC projects that gave what would have otherwise been wayward kids work, pay, supervision, and meaning.
3. People were less desperate than they had been in the dreariest part (1932-1934) of the Great Depression.
4. Sudden improvement in police work. The FBI turned its focus on persons who violated the federal law proscribing interstate flight to avoid prosecution. Such made state borders less porous for offenders like Clyde Barrow and Bonnie Parker who may have lived in one state and committed their crimes in others.
5. Change in public attitudes toward criminals from semi-heroes to arch-villains. Criminals like John Dillinger and Bruno Hauptmann went from being seen as colorful rogues to arch-villains for killing bread-winners who had done nothing wrong -- and small children.
6. Repeal of Prohibition took away one of the cash cows for violent gangsters who enforced their will and territory through murder.
7. A heavy use of capital punishment for murder. Electric chairs and gas chambers were unusually busy.