Given they have nobody else under investigation, my suspicion is that they are doing this to ensure the evidence is inadmissible at any other trial. This would be consistent, given the dude was already supergrass, and no convictions came of that (one assumes it was on purpose).

BONUS: Prison that Epstein was at is also the same one where Whitey Bulger got rubbed out. Interestingly, the new AG Barr was supposedly "in charge" then.