Well, a small car has about 20x the mass of the average human, is much harder, and feels slow when driven at a speed that only the best athletes can reach. It also has uses recognized by society. With any other instrument not intended for killing, i.e. anything but a knife or a gun, it is generally hard work to kill someone. That is how I would explain the gap: non-vehicular homicide suggests malicious intent either by the instrument or the effort.

But I am no legislator.