Why was the Oral law written down?

Eshkolot- until their days there was never a dispute (machloket) among the sages of Israel. They all would say things [exactly] as they were given to Moses at Sinai and they were the first to dispute...this was the first dispute among Israel on the words of the Torah.

In the days of Rabi this Mishna was said- ...When the students of Shamai and Hillel that were before him three generations increased, disputes about Torah proliferated and it became like two Torahs. [this was] because of the yoke of bondage of exile and the decrees that were decreed upon them and because of this they were not able to focus on clarifying the disputes until the days of Rabi, who God gave him the ability to charm Antoninus the King of Rome....and they were able to rest from their troubles and he sent for and gathered all of the students of Israel and until his day there were no organized tractates rather every student that heard something from the mouth of someone greater than him taught it to others and made signs for it and said such and such a law I heard from such and such a person. and when they gathered every person said what they learned and they worked on verifying the reasons for the dispute and understanding who's words are worthy of being kept and they organized the tractates; the words of Nezikin (damages) on their own...the words of Kodashim (holy things) on their own....