Because God has a mission for every disciple, and Paul was a chosen vessel unto me, to bear my name before the Gentiles, and kings, and the children of Israel: (Acts 9:15)
Thus his mission, while primarily to the Gentiles, was to all, and preaching the one gospel of repentance and faith in the crucified and risen Lord Jesus to save sinners, like Peter, whose main mission was to the Jews.
Nor was Paul the only apostle added after Matthias, as
Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out, (Acts 14:14)
Have we not power to lead about a sister, a wife, as well as other apostles , and as the brethren of the Lord, and Cephas? Or I only and Barnabas , have not we power to forbear working? (1 Corinthians 9:5-6)
So Paul will have a throne in the Kingdom when Christ returns?