She isn't under investigation at the moment.  Trump and Sessions have had a lot of distractions to deal with.  But Trump made a campaign promise to prosecute her, and it's important to him and his base that he keep his promises.  There is nothing to prevent him from prosecuting her after she leaves office but a long-standing "tradition" that the incoming administration doesn't prosecute members of the previous one, which is one reason "The Swamp" exists.  If we're going to Drain the Swamp, we have to prosecute the criminals.  Otherwise, they can commit all the crimes they want while their party controls the Justice Department, then walk away Scott-free when they're voted out of office.

Ending that "tradition" will give a heads up for the Republicans in office now, too.  They'll know they can't be protected by a Republican Justice Department, if and when the Republicans are voted out.  It would give us a far less corrupt government than we had under Obama.