Benjamin Franklin and Benjamin Rush were in on the planning. At the time it was the largest and most expensive building in America. It had heating and plumbing when even the president in the Whitehouse - Andrew Jackson - was using fireplaces and chamber pots.
In those days, governments had no way to track or even to positively identify prisoners. No dental records, fingerprints, retinal scans, drivers licenses, DNA samples, not even photographs. Officials had no idea what recidivism rates were. On both sides of the Atlantic, criminal sentences had traditionally been humiliation, torture and/or death, with incarceration merely incidental. Maybe a few hours in the stocks as a target for rotten food and rocks for drunkeness or swearing. Caning or whipping for more serious offenses. Perhaps a tongue cut out for heresy. Hanging or burning for the most heinous crimes like murder or practicing witchcraft.
By the late 1700's, a number of social theories about prison reform were floating around. The Quaker-inspired philosophy was that if criminals had quiet, reflective time and religious instruction they would be moved to penitence and reform. Thus the word "penitentiary".
So a facility with 7'x12' individual soundproof cells was built. Each containing a cot, toilet, and table. Prisoners spent their entire sentences in those cells. Food was delivered via a tiny door. No mail, messages or visitors were permitted. No human contact of any kind was allowed except with religious instructors. Guards in the corridors wore socks over their shoes so that not even footsteps would be heard. Each prisoner was issued a bible. No other reading material was permitted. Each cell had a skylight called the "Eye of God" to remind prisoners that they were being constantly watched.
The system didn't reform criminals or bring them to penitence. It drove them insane.