A memo sent by current Attorney General Jeff Sessions explained that the August 2016 order to end the DOJ's use of private prisons, "impaired the Bureau's ability to meet the future needs of the federal correctional system."
At the time initial order was given, President Obama's deputy attorney general Sally Yates (who would later be removed from office by president Trump) wrote that private prisons "simply do not provide the same level of correctional services, programs, and resources; they do not save substantially on costs; and as noted in a recent report by the Department’s Office of Inspector General, they do not maintain the same level of safety and security."
While the Obama administration's decision to end the relationship between the DOJ and the for-profit prison industry did not include Immigration and Customs Enforcement agency detention centers—and didn't effect the majority of private prisons in America, which operate on the state level—it was heralded at the time as a poisitive step toward reforming the American penal system.
Now, the Department of Justice will resume business with an industry which has been a strong supporter of the Trump administration, and whose stocks skyrocketed following his electoral win. (They spiked again after Thursday's news.) It's a bet that appears to have paid off. Thanks to Trump's so-called "law and order" policies and his pledges to detain untold numbers of undocumented immigrants along the country's southern border, the private prison industry is sitting pretty.