The gemara was transmitted in Aramaic, the language of the masses, so that it would be accessible to all. After all, the goal of study is to understand what has been learned so that it can be incorporated into our lives.
Later, during the Islamic conquests, Aramaic was overtaken by Arabic as the common language of the Middle East. That’s why some of the greatest Jewish works, like those of Rabbi Saadiah Gaon and Maimonides, were written in Arabic.
And so, when the sages wrote their works in Aramaic or Arabic, they weren’t just being practical; they were fulfilling the mitzvah of writing Torah. For after all, the mitzvah is to write Torah in a way that people can learn and understand. If most Jews spoke or understood Aramaic or Arabic, then that was the language to be used to fulfill the mitzvah of writing a Torah (see the Lubavitcher Rebbe, Sichot Kodesh, Kedoshim 5741. See also Likkutei Sichot, vol. 23, pp. 24–25.).