Why does instability in pitch lower trim drag for an aircraft with a tail?
It might be helpful to look at this article from Boldmethod.

The basic idea is that the farther away the center of gravity is from the center of pressure (or center of lift depending on your terminology) the more lift the horizontal stabilizer will have to generate.  The more lift an airfoil has to generate will result in more induced drag.  Trim drag is specifically the induced drag created by the horizontal stabilizer.  The horizontally stabilizer can actually produce both positive (upward) and negative (downward) lift but the negative lift has the even worse affect of requiring the wings to create more lift to compensate for the downward lift as demonstrated in the Boldmethod article.  

Having the center of gravity at the center of pressure also allows the aircraft to be more maneuverable since less force is required from the horizontal stabilizer to initiate a maneuver.  This is why fighter jets tend to be longitudinally unstable and use fly-by-wire to compensate for it.