A cracked sternum, damaged cartilage, and cracked or broken ribs is quite common when doing chest compressions. You have to compress the chest roughly 2" for it to be sufficient to circulate the blood and it takes about 60lbs of force to get that compression. We are told every time we take a class that we're likely to hear and feel the cracking and to not give up. Better alive with broken or cracked ribs than dead.
Until those broken ribs and sternum result in a flail-chest.
I believe there is body cam footage from inside the ambulance. They used a mechanical chest thumper that looked like it was breaking everything. They drove a few blocks away to get away from the crowd. Also I do not think you are supposed to do compressions on someone who is still alive. So asking why the cops didn’t do it may be a moot point.