Nathan ben Meshullam was asked to give a reason for the duration of the present exile, while that of Babylon, which was inflicted upon the Jews as a punishment for the worst of crimes, idolatry, lasted only seventy years.
Nathan ben Meshullam was tasked with justifying the length of the current banishment, whereas that of Babylon, a punishment for the most serious transgression of idolatry, endured only seventy years.