Trump fired him because he was getting to close on the Russia investigation
Trump fired him over the handling of hellary's email scandal. He made the statement just before the election about Weiner receiving secret departmental email communications from Abedin, that were uncovered during the FBI investigation of Weiner's pornographic crime. If he hadn't and this event surfaced, he would have (rightly) been charged criminally with cover-up, collusion and corruption alongside hellary. The ramifications could have had a devastating effect on the DNC.
Why didn't he do it on day one.. and why did he fire the other two
He had just taken over the presidency. He had made other policies higher priority (SCOTUS appointee, healthcare reform, etc., were in need of fixing fast.) This was lower on the to-do list, and is the beginning of the draining of the swamp.
this makes the third person he has fired who were involved in investigating him. he's done nothing on health care or tax reform nor is he building a wall . he has made it easier for banks to fail
this makes the third person he has fired who were involved in investigating him. he's done nothing on health care or tax reform nor is he building a wall . he has made it easier for banks to fail