Maybe because that shot cannot be reached if you tried to stop your momentum with a open stance. You needed the speed thru the hitting zone just to get into position to hit the ball.
Sampras had one of the best.
Just like a batter running to first base, he cannot stop right there, but runs thru past first base.
As opposed to a runner going to third, where he HAS to stop to maintain contact with the bag, he can't get there as quickly.