Yiddish is not just the language of Torah. It is also the language of Yiddish culturalists who would davka have their dinner dances on Yom Kippur and the Yiddish theater. It also contains many vulgar expressions, a nimber of which have crossed over into everyday English.
The only reason Talmud Bavli was written in Aramaic was because that was the everyday language. If it had been written in Arabic ( as the Rambam did becaus that was his everyday language ), would that make it holy? If Yiddish was so holy, why didn’t Rashi, Tosfos, the Ran etc write in Yiddish?