Because the only information that isn’t already public is speculation (a mix of outright wrong and unknown if wrong but unprovable in court), victim and witness names, and maybe some investigative techniques. Releasing them would expose victims and witnesses to doxing, feed conspiracy theory on the speculation (including the known-wrong stuff, but to many people “being in the file” gives it some kind of aura), and let future traffickers know how Epstein was finally taken down so they can run a tighter operation and avoid getting caught.