McCabe held a gathering of senior FBI officials after Trump’s election and mouthed off F Trump several times according to multiple sources. Why he is still employed and why he was made acting director is a large blemish on this administration. All of this is beside the fact that his wife accepted a huge bribe disguised as a campaign contribution just as the FBI began to investigate Hillary. This guy is sleazy and needs to be in jail.
The answer is surprisingly simple and brilliant. Trump and Sessions kept McCabe at FBI where they could watch his every move and listen to his every word - all while McCabe believed he was safe and Mueller was going to nail Trump. Surprise, surprise, surprise!