Why part of the Torah was meant to be transmitted verbally

Hi,

"R' Yehudah haLevi b'R' Shalom said: Moshe requested that mishnah be put in writing as well, but Gd saw that the nations would translate the [written] Torah and read it in Greek and say, "We are Israel, too," and so Gd said, "I will write [only] the majority of My Torah [and so] they will be considered as foreigners." (Hosheia 8:12)"

(Midrash Tanchuma Vayyera 6)

Have a great day,
Mordechai