Why did the apostles stay in Jerusalem instead of going to the Gentiles?
God had not yet permitted them to move past their ministry to Israel. Their calling remained the apostleship of the nation of Israel, so the apostles stayed in Jerusalem rather than turning to the Gentiles, even after Peter's earlier visit to Cornelius and the evidence that Gentiles could be saved.
Les explains that after Peter saw the conversion of pagan Romans at the house of Cornelius, he did not tell the others to leave Jerusalem and go to the Gentiles. Instead, he returned to Jerusalem and was challenged for going to a Gentile, and there they stayed. Les says the reason was that God had not yet permitted them to move any further than their ministry described as the apostleship of the nation of Israel. The context also notes Peter's testimony in Acts 15, where after much disputing he rose up and reminded them that long ago God used him to speak to Gentiles, referring to Cornelius; that had been 12 years earlier.
And when James, Cephas, and John, who seemed to be pillars, perceived the grace that was given unto me, they gave to me and Barnabas the right hands of fellowship; that we should go unto the heathen, and they unto the circumcision.