Albert Barnes on Romans 11:19

Thou wilt say then. Thou who art a Gentile.

The branches were broken off, etc. The Jews were rejected in order that the gospel might be preached to the Gentiles. This would seem to follow from what the apostle had said in Rom 11:11,12. Perhaps it might be said that there was some ground of exultation from the fact that God had rejected his ancient people for the sake of making a way open to admit the Gentiles to the church. The objection is, that the branches were broken off in order that others might be grafted in. To this Paul replies in the next verse, that this was not the reason why they were rejected, but their unbelief was the cause.