The move away from speaking Yiddish as a primary language happened generally during the first half of the 20th century, this change had three main causes. First, and most importantly was the Holocaust. Of the 6 million Jews killed (two of every three Jews in Europe, and one of three Jews worldwide, the Jewish population still has not reached pre 1939 levels) the majority were Yiddish speakers. Poland had one of the largest Jewish populations in Europe, and was considered the center not only of Jewish life but of Yiddish culture, almost all Polish Jews spoke Yiddish as a first or second language. Poland was also the center of the Holocaust, both because of it's enormous Jewish population (3.5 million in 1939) but because it was an area outside of Germany that the Nazis had a strong hold over for nearly the entire war. The Nazis created the largest ghettos in Poland, and all the extermination camps that killed mass numbers of Jews with gas were in Poland. In short the Nazis had determined that Poland would be Europe's Jewish graveyard (USHMM). By the end of the war over 90% (approx. 3 million) Polish Jews had been killed, nearly all of them Yiddish speakers, in fact, Eastern Europe in general had both the highest numbers of Yiddish speaking Jews before the Holocaust and the largest percentage of Holocaust victims, simply because these areas were where the largest numbers of Jews lived.

The second major cause of the downturn in Yiddish speaking in Ashkenazi communities is assimilation, which was generally caused by a combination of antisemitism and economic anxieties. Violent antisemitism and grinding poverty began to push Ashkenazi Jews out of their homes in Eastern Europe and towards Western Europe, Great Britain, Canada, South America, and overwhelmingly to the United States in the late 19th to early 20th century. Between 1881 and 1924 the American Jewish community skyrocketed, almost entirely due to the influx of Yiddish speaking Ashkenazi Jewish immigrants from Eastern Europe. American antisemitism, isolationism, anti-immigrant sentiments and the "American Dream" economics, meant that in order to get out of poverty American Jews had to speak English. For several decades Yiddish continued to be spoken commonly in the community and in the home, Yiddish newspapers and Yiddish theatres were booming. Yet, as fears of antisemitism and anti-immigrant sentiments rose through the 1920s and 1930s people kept Yiddish as something only safe within the community. Then as the native Yiddish speaking generation began to age, and the American born bilingual generation began to move to suburbs and continue to assimilate in the 1950s Yiddish began to be spoken less and less. Holocaust survivors who immigrated in the 1940s and 1950s in many cases wanted to leave their past behind them, and for many of them that included speaking Yiddish outside the home and other familiar safe spaces. By the 1960s Yiddish was spoken primarily in the home as a second language, though Orthodox and Hasidic communities particularly in New York state continued to speak Yiddish as a primary language. Today many Orthodox and Hasidic Jews continue to speak Yiddish as their main language, and Hasidic Yiddish has become in many ways its own dialect. Today Yiddish is primarily spoken in the US by scholars and revivalists known as Yiddishists, elderly immigrants and Holocaust survivors, and certain sects of Hasidic and Orthodox Jews.

The final cause is somewhat similar to assimilation but is primarily focused in Israel. In the early 1900s when political Zionism was beginning, most Jews living in Ottoman Palestine spoke the local dialect of Arabic or their own dialects of Judeo-Arabic. However, most of the young political Zionist immigrants spoke primarily Yiddish. Yet many of them saw Yiddish as a language of the diaspora and a language of oppression. Furthermore, they believed that if Jews were going to establish a large independent community in their ancient homeland they needed a shared language, since at the time there was no single Jewish spoken language. There was however, written Hebrew, which had continued to be the religious and scholarly lingua franca of Jews worldwide. Eliezer Ben-Yehuda, a late 19th century lexicographer is considered the primary reviver of Hebrew as a modern spoken language. There was a very large push among the Jewish community in the British Mandate in Palestine and later the new state of Israel to make modern Hebrew the unifying national language of Israel. Unfortunately this meant assimilation into Hebrew speaking of the tens of thousands of Jewish immigrants into Israel. This didn't just mean Ashkenazi Jews speaking Yiddish less, but meant suppression diaspora languages in general including Yiddish, Ladino, and Judeo-Arabic in favor of Hebrew. This happened in somewhat similar ways to American assimilation, with Hebrew being the official language of things like government and schooling and other diaspora languages being relegated to the home where over the decades they became less prominent.