Either there are still a lot of deep state Clinton supporters left at the DOJ, or there is an active investigation going on and they do not want to release the docs. In the latter case they might not want to tip off HRC about the investigation by citing that the docs cannot be released due to the the investigation. Who knows though with the DOJ.

"Turns out we had to delay today's Clinton email deposition because the State and Justice Departments failed to comply with the court-ordered deadline for needed documents."

https://twitter.com/TomFitton/status/1114280739791691778