To distinguish it from the Chaldean Aramaic. The appearance of Aaramaic goes back as far as 8th century BC. During the Babylonian domination, the Jews began to use Aramaic in official communications and, little by little, it became the dialect of the Jews in Israel, despite considerable resistance of the religiou leaders.
The particular Aramaic dialect spoken by the Jews was the Chaldee (or Chaldean) Aramaic (which the Greeks called hebraïstí), not Syriac (which the Greeks called syrïstí). In the OD, the Greek LXX (Septuagint) refers to (biblical) Hebrew ioudaïstí.
The Jews in the first century AD did not speak Syrian Aramaic but Chaldean Aramaicnot (biblical) Hebrew. Although their differences may have been greater than those in the modern English versions, it is the same as saying that Americans don't speak British English, but American English, but it is English nonetheless.
"Then spake the Chaldeans to the king in Syriack [Aramiyth], O king, live for ever: tell thy servants the dream, and we will shew the interpretation."
Syriac and Aramaic were synonyms for each other from start to finish.