Just speculating, but perhaps the IT person(s) responsible for maintaining the site are nonessential, and considered a simple placeholder safer than leaving the full site online and unattended?
Especially since the USDA receives information from across the country and performs statistical analysis. It would be safer to shutdown the dynamic services rather than let it get hacked. In 4 hours they probably do not have enough time to separate dynamic and static information when they also have to do a backup and a bunch of paperwork.