The “lingua franca” of Jews has always been Hebrew. Yiddish was always for popular literature such as novels, lashon hora, or books for people lacking the skills to read serious Jewish literature which was and is in Hebrew. Even during Yiddish’s “golden age” in the late 19th and early 20th century, most serious Jewish books were written in Hebrew. Remember that the period in which Yiddish-speakers were a majority of Jews lasted a relatively short time (perhaps from the 17th century until the 1940s) – okay, that’s a short time by Jewish standards.
However Yiddish as a living language is an important aspect of the Ashkenazi Hareidi community, but there are very few tools for learning it other than to pick up some children’s books and hang out in neighborhoods where the kids are talking Yiddish to each other. We could probably use some frum materials for learning Yiddish, since the secular (YIVO-style) materials reflect the long dead dialect of the secular Yiddishists (learning it can be useful, however I wouldn’t want to learn Shakespearean English if my goal was to read and speak to modern day Americans).