Bad shot placement is the likely answer, and I can't fault the cops. It's probably very hard to hit a 2' x 3' fast moving target when you're trained to shoot 5-6' tagets in the COM.
And, in a handgun caliber anything but a head or heart/lung shot won't drop a large strong dog very quickly. Dogs (like Rotties) are VERY durable animals and can take a lot of punishment before going down.
And possibly the fact that 4 shots with a 9mm takes as little as 2 seconds, less with multiple officers shooting. Dogs momentum or addrenaline could have been carrying him for the final seconds before it fell.
The 9mm is still a quality round, and the MOST sold individual round in the world.
(That said, I carry a .40)
