Because when you fire a weapon you are responsible for every round you fire. Unlike Hollywood, in real life hitting a moving limb is a difficult shot. It is likely the officer would have missed. There were cars and buildings in the background where errant rounds could have injured or killed innocents.
Additionally, the suspect was already too close. A miss, or even a minor hit probably, almost certainly would not have stopped the assailant. He would have reached the officer and inflicted a wound, potentially a fatal wound, with the weapon he had.
And while the perp was flailing his arms, the officer might have missed and hit someone behind him.
A hit to the center of the target is actually safer for many reasons — stops the perp and avoids collateral damage.