Why did Jesus tell the disciples not to go to the Gentiles?

The passage here is Jesus’ commission to the disciples in Matthew 10.  Jesus tells the disciples to go only to the people of Israel but to stay away from the Gentiles and Samaritans.  This is because during Jesus’ life on earth they were all still a part of the Old Testament and the Dispensation of Law.  The New Testament spiritually does not begin until the death of Jesus (Hebrews 9:15-17).  Before the cross Israel was the primary focus of God.  Jesus was trying to show the nation of Israel that He, the Messiah, had come and they were to follow Him (Deuteronomy 18:15).  It is not that Jesus does not care about the Gentiles or Samaritans here, but that He is following the spiritual preparation for Israel to prepare for the Kingdom of Heaven (i.e. the Millennial Reign).