Why were Jews scattered all over the known world at the time of Acts 2?
Jews were scattered throughout the known world because of earlier dispersions: the Babylonian captivity left most Jews outside the land, with only a few returning, and the ten tribes had been taken into Syria and never came back. Thus, by Acts 2, Jews lived from one end of the empire to the other.
Les explains that at the time of Acts 2, Jews were already dispersed from one end of the Roman empire to the other. The Babylonian captivity had scattered them, and only a few came back to the land. Most stayed out in Babylon and then scattered further. Earlier, the ten tribes had been taken up into Syria, and they never came back. So when Peter speaks of those "afar off," Les understands this as referring to dispersed Jews, not Gentiles—those whom the Lord would call to salvation.