Why could Peter, James, and John only go to the Jews for a few more years?
Because the Romans would overrun Jerusalem within a few years, causing Israel to cease as a viable entity and scattering the people into every nation. Therefore Peter, James, and John could minister to the circumcision only until that judgment fell; their mission was also practically a lost cause due to Israel's unbelief.
Les explains that after Paul and Barnabas received the right hands of fellowship, it was agreed that Paul and Barnabas should go to the heathen, the Gentiles, while Peter, James, and John would go to the Jew, the circumcision. Yet Les notes they could only go to the Jew a few more years, because the Romans would overrun Jerusalem, Israel would cease to be a viable entity, and the people would be scattered into every nation on earth. For those next few years, Peter, James, and John would hold forth among the children of Israel, but Les says it was practically a lost cause because of Israel's unbelief. That is also why Paul constantly had to defend his apostleship.