The pedestrian ultimately got hit because a lot of fallback systems failed: They disabled the software's ability to emergency-break (1), they disabled the cars' systems to emergency break, and the operator of the vehicle was distracted, not reacting in time.

(1) This was actually done due to false-positives, i.e. the car slamming on the breaks in non-emergency cases. (I find that's a relevant point, because without that detail it just sounds more ludicrous than it already is).