I think processing freeze and flight or fight syndrome are the best explanations. In the end, many people imagine themselves as acting quickly and heroically, but few people actually do.Watch a therapist’s video, who made some points about why people may not respond to violence.
Another factor in most people not reacting, maybe lack of experience with violence.
Ironically, one civilian population that I have personally seen to have the ability "read" people's demeanor, body language and hand movements- then react rapidly to violence using full defensive violent force are, well...... prisoners.
Long exposure / experience with violence combined with living in environments where survival can depend on having threat radars always up and running, combined with likely Uhhmm...... "personal experience" in using violence can lead to very rapid reactions.
In the end, I suspect that one of the very few groups of people capable of processing what they were seeing, then intervening with decisive force and saving that poor victim would have been former prison inmates.