There are plenty of non-exogenous causes, they just don't have the same cool-factor for publication as things like leaded gasoline. For example many people were released from institutions during the massive de-institutionalization push that started in the mid 50s [1], going from ~340 institutionalized people per 100,000 in 1955 to about 60 per 100,000 in 1980. One can even argue that the sum of incarcerated and institutionalized has averaged roughly constant (about 450 per 100,000) with a dip during the period of increasing criminality from the 1960s to the 1990s as the incarceration rate had to climb from ~100 to ~450 per 100,000 to make up for the mass de-institutionalization.
The cited study is pretty interesting:
"The juxtaposition of these trends and the current high incidence of severe mental illness among those behind bars begs the question of whether the mentally ill have simply been transinstitutionalized from mental hospitals to prisons and jails. A related question concerns the extent to which the unprecedented growth in incarceration since the late 1970s is driven by a reduction in public investment in inpatient mental health services. Past changes in sentencing and corrections policies are currently under heightened scrutiny as state prison populations are at record levels and many states are seeking to scale back correctional populations with an eye on the fiscal benefits of doing so. To the extent that the run-up in state prison populations was driven by deinstitutionalization, the current focus on sentence enhancements and the evolution of the U.S. sentencing regime may be misplaced."
If crack was a big driver for the previous crime wave, it heroin/fentanyl is likely to be a driver for this one. Or, on the West Coast, meth. It’s made a pretty big comeback since 2017, but it’s been mostly overshadowed by the opioid crisis.
On the other hand, this latest round seems to be the result of poor policy choices.