He allows himself to be accosted by the truth of an Other, to be brought to the brink of transformation. Maybe you can see aspects of yourself in the victim, the man who fell into the hands of robbers. The road to Jericho was, and still is , notoriously dangerous. But clearly, he is undeserving. He's overcome by the senseless evil of this world and by circumstances beyond his control.
Bad stuff happens to good people.
What's intriguing in the story, although he's cast as powerless, he's actually the one who gathers the community. He is the catalyst for their conversion. We're not sure what drives them to commit this violence. I tend to think that the latter answers make more sense because not only do they rob the victim, but they strip him and beat him. They take advantage of his vulnerability.