Liability comes to mind. Contractors are expected to have the necessary safety equipment, specialized training and knowledge and they are covered by their own insurance. Volunteers might have the enthusiasm, but don’t always have the necessary equipment or more important, skills and knowledge.
Considering the scope of work the EBT owners appear to have planned, it also might be better to use volunteers where they’re better suited. Cleaning up the grounds & buildings, rehabilitating equipment, lots of painting, etc. There’s no shortage of tasks necessary to reopen.