Why was Alexander Acosta never charged?

This still confuses me. The US Attorney who signed off on the NPA became Secretary of Labor, then resigned in disgrace when the deal became public again. But he was never charged with anything. Why? The DOJ OPR investigation found "professional misconduct" but recommended no criminal referral. The report is partially public (DOJ-OPR-2020-001). Is there a legal theory under which his actions were criminal? Or was this just prosecutorial discretion that we don't like but isn't illegal?