Why did Jesus tell his disciples not to go to the Gentiles before his death and resurrection?
Until Jesus had experienced death, burial, and resurrection, He could not be the object of faith for the mass of the Gentile world. The Jonah type had to be fulfilled first: only after resurrection could the message go to Gentiles, so the earlier restriction awaited the finished work of the cross.
Les explains from the Jonah type that Jonah was not a fit servant until he had fulfilled the picture of death, burial, and resurrection. After the fish pitched him back on shore—a type of resurrection—Jonah could go to the Gentiles, and Nineveh repented. Likewise, Jesus says that until He experienced death, burial, and resurrection, He could not be the object of faith for the mass of the Gentile world. The context ties this to Ephesians 2: Gentiles were once far off but are made nigh by the blood of Christ, not by the Law or covenants. Israel held the covenant promises while Gentiles had no part, but when Israel refused and rejected it, God set them aside temporarily and turned to the Gentile in grace.