In Jesus' day, the Samaritans were a mixed society that had a very tense relationship with the Jews. As a result, Jesus preaching to Samaritans and using them in his parables was extraordinary in that atmosphere.
The Samaritan culture began when foreigners were sent by the Assyrian Empire to inhabit Samaria in approximately 723 B.C. The foreigners intermarried with the remaining Israelites in the area and adopted some of their customs while retaining elements of their native religions. Jews viewed this as a corruption of their religion, for which they harbored hatred towards the Samaritans. Samaritans returned their hatred in kind on various occasions; for example, Samaritans were one of the peoples attempting to halt the reconstruction of Jerusalem's walls. Later, the Samaritan religion rejected most of the Hebrew Scriptures and only accepted Moses' writings, further distancing themselves from Jewish tradition.
By Jesus' day, Jews viewed Samaritans with such disdain that they used the term as an insult towards Jesus in John 8:48. This could also derive from Jesus' childhood being in Nazareth, a town near Mount Gerizim, where Samaritans worshipped. Similarly, the Samaritan woman in John 4:9 was shocked that a Jew would converse with her peaceably. Jesus' usage of the good Samaritan parable was probably done purposefully to put the law expert in an uncomfortable position and see if he was willing to give credit to the Samaritan.