Because the Jewish people, by and large, have refused to accept the gospel, it has gone to the Gentiles.
In the three chapters of Romans 9-11, Paul seeks to figure out why the Jewish people (with a few exceptions, such as himself) have not accepted the gospel, but Gentiles have. During his time the Christian faith was becoming increasingly a Gentile religion in most parts of the Roman Empire, including Rome itself. Paul claims that God has made a temporary detour. For the time being, Israel's "no" to the gospel is in accord with God's plan. The "no" allows the gospel to go to the Gentiles, where it is being received. Salvation has come to the Gentiles, he says, but he also expects that the growth of Christian faith among Gentiles will make the Jewish people "jealous" enough to reconsider.
The Salvation of the Gentiles
11 So I ask, have they stumbled so as to fall? By no means! But through their stumbling* salvation has come to the Gentiles, so as to make Israel* jealous.