The root cause is that a huge portion of the US population sees prisons entirely from a punitive point of view. Want to reduce prison sentences? Improve conditions? You're soft on crime!
It seems like there might be room for improving conditions in ways that will help prevent them from being back in prison later. Education and working in a way that develops skills are two ways that come to mind.
Even if you can prove a non-profit, non-funded program reduces recidivism, if it also happens to make prisoners’ lives marginally better in the process, that’s an immediate no.