The Oral Torah wasn't supposed to be written down because it is of less importance than the actual Torah and the Jewish leaders didn't want people to lose the focus. There was intense persecution though and because of that, the Talmud was compiled so it would not be lost. 

Here are words from: Pirkei Avot (Ethics of the Fathers).

[3] Antigonus, leader of Socho, received the tradition from Shimon the Righteous. He used to say: "Be not like servants who serve their master for the sake of receiving a reward; instead be like servants who serve their master not for the sake of receiving a reward. And let the awe of Heaven be upon you."