It looks like it might happen in the future, but how has it not happened by this point? I realize that it's very difficult to impeach a President, but after his whole Tweet (Can that lead to an Impeachment?) about Obama wiretapping his phones, couldn't that have done it? Can Obama file a suit against him or something? I'm really not sure how something hasn't happened, or how Trump hasn't had to testify about something at this point. And how can Trump fire the person who is investigating him at this time? Something seems really off about this.
There is an excellent reason why Trump hasn't been impeached yet. He hasn't been tried and convicted of committing any high crimes or misdemeanors.
That is the requirement for impeachment under Article 2, section 4 of the U.S. Constitution.