I don’t know anything for sure, but if he were a registered confidential informant with some agency they could shut down investigations and prosecutions for him

Related personal theory that I have no evidence for - Donald Trump was turned into an FBI asset sometime in the late '80s or early '90s to get them info on Russian mobsters that were running around NYC in his circles but he flipped on them at some point and they decided to sweep the whole thing under the rug rather than air their dirty laundry