The Samaritans were seen as half-Jews. When the Assyrians dragged the Jews off into captivity, some were left behind. People of other nations were brought in to populate the region (II Kings 17:24). When they struggled, the Assyrians sent Levites back to the area to teach the people how to worship the God of that land because they had a strong belief that various gods ruled various areas (II Kings 17:26-28; Ezra 4:2).
Thus, you had a group of people that had some Jewish blood in them. But because they were seen as corrupted Jews, the full-blooded Jews would have nothing to do with them (John 4:9). This resulted in the Samaritans establishing their own place of worship (John 4:19).
Therefore, while it was evident that Christianity was not limited to the Jews, there still needed to be a push to prove that Gentiles without any trace of Jewish blood could be accepted as Christians.
Thank you for the clarification.