A very good question. Another question is why does Comey and his deputy still hold their jobs at the FBI.
Blackmail? Maybe Koskinen would comment on the ongoing audit of Trump when he leaves government. The laws make such comments more circumscribed being a government official.
I think Comey will be hard to get rid ot. He has a 10 year contract and would probably have to get charged and convicted of something to be removed.