Acts 18:5-7 5When Silas and Timothy came from Macedonia, Paul devoted himself exclusively to preaching, testifying to the Jews that Jesus was the Christ. 6But when the Jews opposed Paul and became abusive, he shook out his clothes in protest and said to them, "Your blood be on your own heads! I am clear of my responsibility. From now on I will go to the Gentiles." 7Then Paul left the synagogue and went next door to the house of Titius Justus, a worshiper of God.
Because he did....
Romans 15:15 I have written you quite boldly on some points, as if to remind you of them again, because of the grace God gave me 16to be a minister of Christ Jesus to the Gentiles with the priestly duty of proclaiming the gospel of God, so that the Gentiles might become an offering acceptable to God, sanctified by the Holy Spirit.
Paul was apparently called to be an apostle to the Gentiles AFTER his first missionary journey,.
Galatians 2:1 Fourteen years later I went up again to Jerusalem, this time with Barnabas. I took Titus along also. 2I went in response to a revelation and set before them the gospel that I preach among the Gentiles.
Galatians 2:7 On the contrary, they saw that I had been entrusted with the task of preaching the gospel to the Gentiles, just as Peter had been to the Jews. 8For God, who was at work in the ministry of Peter as an apostle to the Jews, was also at work in my ministry as an apostle to the Gentiles. 9James, Peter and John, those reputed to be pillars, gave me and Barnabas the right hand of fellowship when they recognized the grace given to me. They agreed that we should go to the Gentiles, and they to the Jews.
Well after. I do not disparage Paul in the least; but let us look objectively at the roles that the individual Apostles played.