Shootability was not a part of that test, neither was there a size/weight restriction. But there were a number of other requirements and HK apparently went with a huge, heavy gun to meet those requirements.
You may note that when they started again for a pistol that was going to be tested differently (by a unit known for its handgun skills and used to shooting 1911s, in fact) that they made the gun a lot lighter, especially in the slide.
But they still stuck with the basic USP/P2000 design because they haven't created a gun out of whole cloth since the USP40 in what, '93?