He would have to know he's safe to stick his neck out as far as he has so far. Either he knows that nothing illicit occurred, or he knows that the evidence is out of the wrong hands. Maybe Epstein clogging his tunnels with cement might have emboldened Dershowitz.
Besides that, Epstein would basically be nailing himself if he released a video of Dershowitz screwing a young girl on his island…but I'm guessing that Dershowitz knows that nothing incriminating exists. He's too crafty for that shit.