ames,
Yiddish is used by the Gedolim and has a holyness to it (certainly less than Loshon HaKodesh) since it was used by Yidden for 1,000+ years to learn Torah. It also allows Yidden from across the Globe to communicate with one another, despite other linguistical and cultural differences. And this is why in (many) Yeshiva’s the Rosh Yeshiva’s shiur, if not all shiurim, are given in Yiddish and it is used as the primary language by the Gedolim and Rabbonim.
(BTW “ivrit” is a treif language invented by a secular Torah despising zionist.)