President Trump would not do this, unless he’s become apprised of information that makes prosecuting Hillary a moot point.
If, for instance, he’s discovered that she has only six months left to live, he might choose to call off the dogs, in order to spare the nation from a process that will most assuredly create even deeper divisions amongst us.
Perhaps he’s discovered details about her and Slick’s relationships with world class baddies, that indicates to him that the Clintons are about to be punished far worse than anything U.S. justice could mete out.
Maybe he’s discovered that the rabbit hole goes much, much deeper than any of us have imagined, and that the Clintons are actually minor players in a much bigger game.
The true scale and depth of the rot and corruption may go so deep, that he’s being forced to make this choice as a strategic move, in order to successfully wage an even greater battle after he’s sworn in.
Whatever’s going on, we’re not even seeing the tip of the iceberg.
...or maybe Trump lied about going after her in the first place.
(The answer that requires the fewest assumptions is usually the correct one)