Why is Saint Paul the Apostle to the gentiles?

Because he was sent to preach to the gentiles.

Additonal Answer:

Paul's mission was threefold - the Gentiles being listed first, then Kings, and then the 'children of Israel' - includes the lost 10 tribes:

Acts 9:11-15New King James Version (NKJV)

11 So the Lord said to him, "Arise and go to the street called Straight, and inquire at the house of Judas for one called Saul of Tarsus, for behold, he is praying. 12 And in a vision he has seen a man named Ananias coming in and putting his hand on him, so that he might receive his sight."

13 Then Ananias answered, "Lord, I have heard from many about this man, how much harm he has done to Your saints in Jerusalem. 14 And here he has authority from the chief priests to bind all who call on Your name."

15 But the Lord said to him, "Go, for he is a chosen vessel of Mine to bear My name before Gentiles, kings, and the children of Israel