That was how their rebbeim taught them, and how the rebbeim of their
rebbeim taught them, going back many generations. It’s not
like they decided one day to create an entry barrier…
And it’s not like there aren’t any Torah classes in English.
Moshe taught the Torah in Yiddish to Yehoshua , Yehusua taught it to the Elders in yiddish , the Elders to the Prophets in yiddish , and the Prophets gave it over to the Men of the Great Assembly in yiddish