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.
Because anyone (everyone) on that list is going to immediately sue for defamation etc, and if they don’t have evidence or they messed up the chain of custody on that evidence, it’ll be an issue. They also will be able to look at all the evidence through discovery and see if it’s tampered with. So that’s why they can’t release even 1 name, because they are protecting Trump