Having understanding of the Bible tells you why Jesus told his disciples to not go to the Gentiles.
The New Covenant had not happened yet because Jesus was still alive and therefore under the Old Testament. The Jews, based on the Old Testament law that existed were not to associate with Gentiles. Jesus could not break His own law.
However, after Jesus death, Gentiles could be saved under the New Testament. That was the reason for Jesus giving His life, to die for the sins of the world, to include Gentiles. After His death He went to Apostle Paul to take the Good News to the Gentiles.
Now how many Gentiles do you think have been saved from the time of Jesus death on the cross some 2000 years ago compared to the few you think were not saved. That was a short span of time from the time Jesus told his disciples to not go to the Gentiles to the time of His death on the cross, so under the New Testament they were able to be saved after Jesus died.
Now you are asking why the King of Kings and Lord of Lord did not save Gentiles during the time He was alive on earth, when that was His purpose all along. He came to die for us all, Jews and Gentiles.