According to wikipedia, portraying a Samaritan as good was a shocking chidush. That story in the Christian bible was meant to be provocative, not normative.
In other words, the parable of one good Samaritan is to the exclusion of all others who were not.
Furthermore, it was a parable. Maybe that guy never really existed.
The story specifically used a Samaritan because they were at odds with the Pharisees. He could have used as an example an Israelite who wasn’t worried about becoming Tamei Meis but chose a Samaritan to make a point.