He's been fired for his mishandling of the Clinton email investigation. . .
Bannon's got chutzpah, you gotta give him that.
edit: Ok, more seriously recent testimony is being falsely interpreted to justify demands that he should have prosecuted members of the Clinton campaign and his failure to do so is the reason he's been fired. So this actually plays into the unhappiness from last summer where lots of rank-and-file FBI were upset he wasn't doing so. Trump has "just now" noticed that discontent and has concluded Comey can't effectively lead the FBI.
Whether it actually is the reason or not, it will have a chilling effect on the Russia investigation and I think it's likely the GOP will let him give it to a toady which means things will go absolutely nowhere. I hate saying it since establishing one is guaranteed to go off the rails but we now need a special prosecutor.