Why he chose to keep the Greeks I can only guess but the hellenised world of the Roman Empire the Lingua Franca of koine Greek did provide a highly specific global language for the gospel to be spread to the gentile Nations.
That is not to say these nations were wholesome infact the Greeks are mentioned in Daniel 10 and the power behind them is confronted by angelic beings
God has a decree he will raise individuals and nations to prosecute his plan throughout history and into the future
English Standard Version
Then he said, “Do you know why I have come to you? But now I will return to fight against the prince of Persia; and when I go out, behold, the prince of Greece will come.
