"For hundreds and thousands of years, the Oral Torah (as the name implies) had consisted of verbal transmission of the tradition from master to student, with nothing committed to writing.
The tradition passed from one Beit Midrash to another, which from generation to generation changed their character and methods of study.
One element, however, remained stable throughout: the tradition was oral, not written.
It is true that in earlier times, and even during the
As a rule, the Oral Torah was not written down.
These scrolls, known as 'hidden scrolls' and not meant for public use, were neither studied nor used for teaching, and were preserved only as the personal memoranda of individual Sages.
Moreover, there was a halachic ruling to the effect that 'the words which are written, you are not at liberty to say by heart, and the words transmitted orally, you are not at liberty to commit to writing' (Gittin 60b).
One reason given for this was that an oral doctrine enables maximum flexibility in transmission and interpretation, whereas a written text is bound to reach, at a certain stage, a point of ossification beyond which it cannot be developed.
Exposition of a written text becomes by nature supplemental, while the text itself is no longer renewed and invigorated.
Thus, alongside the written Torah there coexisted a more flexible tradition, which conveyed a practical understanding of the Torah's basic terms and concepts and, above all, explained the actual practice of its commandments.
All this had been transmitted in an ancient chain of tradition stretching from Moses through the whole list of Sages detailed in Pirkei Avot (chapters 1-2) until Rabbi Yehudah HaNasi's (Rebbi's) generation.
This heritage was zealously preserved as an oral tradition, not to be recorded, not to be petrified.
Despite these and many other considerations, Rebbi decided that the time had come to change the method of preserving the Oral Torah by establishing hard and fast rules for guiding its interpretation and formulating it in a specific, clearly defined way that would meet the needs of the time."
–Rabbi Adin Steinsaltz