Trump is probably on the list, so he has to protect that. The Democrats had it during their entire presidency; even if they didn’t want turmoil during their time in office, they could have released it after their presidential defeat and before Trump took office.
The obvious answer is that the Clintons are heavily implicated, but in a more general sense, the point of blackmail is to extort someone into doing something on the condition that you don’t release the blackmail. That’s why they got Epstein to connect with so many academics, celebrities, and other public figures. If they were to just release the blackmail, it would lose intelligence agencies all their leverage.