Good point, I was wondering the same thing. According to the Washington Times his case was referred back to the Judiciary committee back in December, where it died.
Maybe he'll be charged too. Impeachment didn't work out (2/3 senate majority required even if it made it to the senate, which it didn't, never could happen), but now that Sessions is at DOJ, he can be charged with crimes. What crimes, I don't know -- obstruction of justice? They couldn't agree in the impeachment hearings about what he was actually responsible for. But I remember thinking how arrogant this creep was, what a radical partisan he was, and how much contempt he had for the committee during the hearings.
Gave 100,000 dollars to Democrats by 2014:
http://www.foxnews.com/politics/2014/06/25/irs-commissioner-john-koskinen-is-major-democratic-donor.html
Washington Times:House Republicans derail impeachment effort against IRS commissioner
http://m.washingtontimes.com/news/2016/dec/6/john-koskinen-irs-commissioner-spared-impeachment-/
Maybe Trump does not care about his politics. As long as he is bringing in record tax dollars Trump may not care. Same thing would happen in a large corp.
Obstruction would be a good start. Lying under oath would be another.
He needs to go now. He needs to be charged asap. I hope Sessions has it in him.