Why did Paul say 'from henceforth we go to the Gentiles'?

Paul declared this because Israel as a nation was rejecting the Gospel after roughly thirty years of ministry, even though they were the covenant people and the synagogue was the logical starting place. Therefore the precious Gospel turned primarily to the Gentiles, though Jews may still come the same way by faith.

Les explains that Paul’s pattern in Acts was to go first to the synagogue of the Jew, because Israel had been the covenant people, steeped in the Word of God, and the very writers of Scripture. It was a logical place to start. But after about thirty some years, as they continued to reject the message, Paul reaches the final statement, “from henceforth we go to the Gentiles,” because Israel as a nation was rejecting it. For the last 1900 and some years, the Gospel has gone primarily to the Gentiles, but it is still open to a Jew, who must come the same way, since Paul says there is no difference.

When and why did Paul turn from going to the Jews first and go to the Gentiles?

After about thirty years of taking the Gospel to the synagogue first, Paul declared, 'From henceforth we go to the Gentiles.' Israel as a nation had continued to reject the message, so the Gospel then went primarily to the Gentiles, though it remains open to any Jew who believes.