Why was it not unusual for early Jewish believers to preach to Grecians?
It was not unusual because a Grecian was a Jew who was a citizen of another nation outside Palestine, not a Gentile Greek. Jewish believers were accustomed to speaking to fellow Jews, so approaching Grecians fit their existing practice; reaching actual Greeks would have been the real change in venue.
Les explains that the term “Grecian” refers to a Jew who was a citizen of some other nation except Palestine, a Jew who was not a native of Palestine, whereas a Greek is a Gentile. Since these early Jewish believers had been speaking to fellow Jews, approaching Grecians was not at all unusual; the context even mentions Grecian widows back in chapter 6. Approaching actual Greeks, by contrast, was a change in venue and modus operandi, because they had not been talking to Greeks before. This shift fits God moving toward the Gentile world: He started when He saved Saul, then sent Peter to Cornelius, and now Saul had returned from three years of learning and the revelation of the mysteries. These Jews who had preached to Jews only began speaking to Greeks, preaching the Lord Jesus, and the hand of the Lord was with them, so a great number believed and turned to the Lord.