I suppose because the average cat is not that big, that it doesn't affect traffic safety if it is knocked on the road, whereas the animals you have to report could, which is why they are notifying to the police. This is just a guess.
Although I have signed the petition, I can't see it becoming law. Plain human courtesy and decency and caring should be all that is needed, but some people just don't have it. If I knocked a cat I would stop anyway to see if was dead or not, even if there were no houses close by. I would hate to think it was lying there suffering. If it were dead and there was nowhere to enquire, I would remove it, if necessary from the road. It is distressing to others to see dead cats in the road.
The thought of my animals being just left on the road with no regard is horrific.