Why did Jesus tell his 12 disciples not to go to the gentiles and samaritans?

While Jesus was on the earth, His mission was only to "the lost sheep of the house of Israel" (Matthew 15:24). Thus, while He was physically present in the world, he sent the 12 (and also the seventy - Luke 10:1) only to the Israelites.

Only after He was glorified, and the Holy Spirit imparted to the apostles, was the gospel to be preached to "all the nations" (Matthew 28:19; Mark 13:10; Luke 24:46, 47).