Despite significant scholarship, research, and funding dedicated to implementing criminal diversion programs over the past two decades, persons with serious mental illness and substance use disorders remain substantially overrepresented in United States jails and prisons. Why are so many U.S. adults with behavioral health problems incarcerated instead of receiving treatment and other support to recover in the community? In this paper, we explore this persistent problem within the context of “relentless unmet need” in U.S. behavioral health (Alegría et al., 2021).