I assume that the Tanaaim and Amoraim in Eretz Yisrael spoke Hebrew, so why is the Talmud Yershalmi in Aramic not Hebrew?
Actually, it seems that the spoken language of the time was Aramaic. Even in the Mishna, written while the Jewish people were still in Israel, often when document are quoted, it is usually in Aramaic.
Consider, for example, Megillas Taanis, which was written as a calendar of sorts for the nation at large, written by early Tannaim (Shabbos 13a) and entirely in Aramaic. It seems that scholarship was in Hebrew, but the common folk spoke Aramaic.