The widespread adoption of Yiddish centuries ago arose from several unique factors:
a. One has to ask, why would Jews living in the non-Germanic lands of Central and Eastern suddenly, en masse, decide to learn and speak a new language based upon a variant of medieval German? Because they brought what would become Yiddish with them. In the 14th century, renewed persecutions and atrocities drove many German Jews east to Poland ( which at the time controlled much of Eastern Europe). The Jews were still Germans by outlook and viewed their new Slavic neighbors as below them and continued to speak their native German rather than adopt the despised Slavic languages.
b. Because, for centuries, secular governments did not interfere with the internal business of the Jews( as long as taxes were paid) most Jews never had to learn the native language except as needed for commerce.
The same held true, until recently, in the U.S.
People in the Lower East Side, Little Italy, Chinatown etc never needed to speak anything but Yiddish, Italian or Chinese because they never had to interact with outsiders. Today, most storesigns in Flushing Queens are in Chinese