First of all, the knee or leg if difficult to shoot. Police are trained to shoot at center mass which is from the neck down to the hip as a first resort. In case you were not aware, there are very stressful moment when you have a gun pointed at someone who wants to stab you or someone else. Aiming a pistol is not the most easy thing to do contrary to all the Dirty Harry movies you've probably seen.

Another reason is that more times than not, shooting someone in the center mass will stop the threat. Shooting someone in the leg will not stop someone who is determined to stab you. If shooting someone in center mass does not stop the threat then the next option is the head.