ELI5: Why are violent repeat offenders allowed back on the streets?

Why are people with a violent history—like the SoHo stabbing suspect—out on the streets again?
(Article: amNY link)

Genuinely not trying to be political here—I’m just trying to understand how and why this kind of thing happens. Is it a lack of resources? A miscalculation in assessing someone's risk of reoffending? Are we seeing the effects of broader priorities in the criminal justice system—like saving taxpayer money on incarceration and rehab, even if that carries risks? Are the laws just poorly designed? Or is it tied to how difficult it is to commit someone involuntarily to a psychiatric facility?

Curious what people who know more about this think. Thanks!