In general, oral Torah isn't allowed to be written down (Gittin 60b).
For all generations until the Amorayim (at least according to Rashi), that Halacha was kept.
Yet, according to Rashi, Megillas Taanis (generally, a list of days one isn't allowed to fast) was written down much earlier, and was the first part of Oral Torah written down!
Couldn't it be kept memorized like all other Mishnayos? Why was it written down?