When a human kills someone with a car it is almost always in a way we can empathize. When you’re around cars you have pretty good mental models for the humans that drive them and how the car will behave.
When you’re around human piloted cars you can look at the driver and get a pretty good idea of intent. You can tell if the driver sees you, you can tell what their mental state is, if they’re paying attention, what they intend to do. You can sum up a person with a glance, this is the power of evolution, we’re really good at figuring things out about other living things.
Crossing the street in front of a car is a leap of faith, not troubling at all when there’s a human there, but a robot? There’s no body posture, no gestures, no facial expressions, nothing to go on. There’s a computer in control of a powerful heavy machine that you’re just expected to trust.
Robot cars don’t make human mistakes, they make alien mistakes like running down kids on the sidewalk in broad daylight, things which don’t make any sense at all that make people feel like they aren’t anywhere safe.
It won’t take but a couple of cute kids killed in a surprising matter to shut down the whole autonomous experiment.