Why could they only go to the Jews for a few more years?
They could go only to the Jews for a few more years because the Romans would soon overrun Jerusalem. Israel would then cease to be a viable entity and be scattered into every nation on earth. For those few years, Peter, James, and John would hold forth among the children of Israel.
Les explains that the agreement at Jerusalem was that Paul and Barnabas would go to the heathen, that is, the Gentiles, while James, Peter, and John would go to the circumcision, the Jews. But that Jewish outreach could last only a few more years, because the Romans would overrun Jerusalem and Israel would cease to be a viable entity and be scattered into every nation. Yet for those few years, Peter, James, and John would hold forth among the children of Israel. Les notes it was practically a lost cause because of Israel's unbelief, which is exactly why Paul had to constantly defend his apostleship.