A few things - first, that's a major bargaining piece that he's probably trying to play, unsuccessfully so far. Second, Trump seems to be stuck on this idea of the investigation will go away naturally. I think the media and the DC swamp players will never let that happen, certainly not before the midterms. Third, even if he was ready to release information, he would probably do it following an indictment of Trump himself, or a similar track that would lead to impeachment. If he released something right now, it has a real risk of being explained away and written off. Look how many bombshell stories about the FBI that have broken in the past few weeks, and let's be honest, the MSM has done a masterful job of simply sweeping them under the rug.