The US pays private prisons per prison, not per prisoner. So it looks bad if XYZ agency is paying for a prison with empty beds. The incentive/political optics is to fill those beds. Something resource utilization rates something something.

https://en.wikipedia.org/wiki/Northwest_Detention_Center "In October 2015, the GEO Group renewed a ten-year contract with ICE.[11] The contract pays GEO for almost 1,200 beds daily, regardless if they are used."