The scripture verse I cited was Matthew 27:51
And behold, the curtain of the temple was torn in two, from top to bottom; and the earth shook, and the rocks were split;
As I'm sure you are aware, the temple veil (or curtain) was what separated the Holy of Holies from the rest of the temple.
The significance of that event represented the beginning of a new covenant God had with man. Jesus' sacrifice at Calvary was the blood sacrifice that sealed that covenant.
You remember back in Genesis 17 when God said, "Behold, my covenant is with you, and you shall be the father of a multitude of nations."
Well, at that point in time is where God kept that promise.
Hebrews 9 and 10 show a wonderful parallel between the old covenant and the new covenant. The old covenant, as you so rightly point out, was made available only to the circumcision. However, the new covenant is made available to all, as we are circumcised with a circumcision made without hands (Col 2:11)
The bottom line is that before Christ our Passover (pascha, the lamb) was sacrificed for us (1 Cor 5:7), it would have only been appropriate to send the 12 or the 70 to the tribes of Israel. But, since the new covenant was put into place by God's grace, the guidance provided to Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit,(Matt 28:19) is entirely appropriate. Not just for Paul, but for all the apostles.
BTW, one thing to keep in mind is that Luke accompanied Paul on his journeys. Luke was from Troas. As evidence of that, you'll note that the first person starts to be used in Acts 16:11 (Setting sail therefore from Tro'as, we made a direct voyage to Sam'othrace, and the following day to Ne-ap'olis,) -- it is always the third person prior to that point. Thus it is natural that Luke would chronicle the travels and works of Paul. Doesn't mean that the other apostles did nothing or stayed strictly in Israel...it just means that Luke captured what he was told prior to joining Paul and then what he saw and heard after that point.