Mishneh Torah — Transmission of the Oral Law מסירת תורה שבעל פה, Chapter 15

וְלָמָּה עָשָׂה רַבֵּנוּ הַקָּדוֹשׁ כָּךְ, וְלֹא הִנִּיחַ הַדָּבָר כְּמוֹת שֶׁהָיָה – לְפִי שֶׁרָאָה שֶׁהַתַּלְמִידִים מִתְמַעֲטִים וְהוֹלְכִים, וְהַצָּרוֹת מִתְחַדְּשׁוֹת וּבָאוֹת, וּמַמְלֶכֶת הָרִשְׁעָה פּוֹשֶׁטֶת בָּעוֹלָם וּמִתְגַּבֶּרֶת, וְיִשְׂרָאֵל מִתְגַּלְגְּלִים וְהוֹלְכִים לַקְּצָווֹת: חִבַּר חִבּוּר אֶחָד לִהְיוֹת בְּיַד כֻּלָּם, כְּדֵי שֶׁיִּלְמְדוּהוּ בִּמְהֵרָה וְלֹא יִשָּׁכַח; וְיָשַׁב כָּל יָמָיו הוּא וּבֵית דִּינוֹ, וְלִמַּד הַמִּשְׁנָה בָּרַבִּים.

Why did Rabbenu Hakadosh make [such an innovation] instead of perpetuating the status quo? Because he saw the students becoming fewer, new difficulties constantly arising, the Roman Empire spreading itself throughout the world and becoming more powerful, and the Jewish people wandering and becoming dispersed to the far ends of the world. [Therefore,] he composed a single text that would be available to everyone, so that it could be studied quickly and would not be forgotten. Throughout his entire life, he and his court taught the Mishnah to the masses.